I'm been rattling this question around in my head for a bit: in which season was each of the characters at their best? (And worst?)
Obviously this is just my opinion but I am interested to hear yours as well!
*I have based my rankings around how enjoyable the character was for me to watch
Worst: Season 5
"Exposed" for me encapsulates all the problems with this character. Jonathan defends Old Friend against charges of having flings and going to strip clubs, deciding it's all Lex's fault somehow. When it turns out Old Friend is doing all that stuff, it's still all Lex's fault, even though he assisted Clark in stopping a killer. Therefore Jonathan must run against Lex for office, kicking off a very boring plotline.
Best: Season 2
I found Jonathan much more human and less speechy in S2. Favourite moments: the wonderful scene with Clark at the end of Rosetta, where Jonathan for once doesn't have a pat answer; the revelation in Lineage that Jonathan was willing to make a deal with Lionel, thus screwing over the Rosses; the banter with the new Sheriff, highly unimpressed with good ol' boys of Smallville and most of all this perfect line (from Rush): "And tell Pete to stop cutting donuts out in the field. He's scaring the cattle!" Best episode: Rosetta.
Worst: Season 4
While I didn't like Red Queen Martha in S9, her worst full season by far was S4. Yes, Annette got to sing "La La" one time but that didn't make up for suddenly working for the Talon (?) accepting relationship advice from Lana (??) and worst of all, tearing a strip off a weeping Clark for marrying Alicia on Red K. Nasty stuff.
Best: Season 2
The baby plot put an end to the good times, but while the Luthor plot lasted it was fun to watch AOT get off the farm and interact with the Luthors. Lineage is a lovely Clark/Martha story. And Clark frantically appealing to "Mom!!!" when Jonathan won't let him go on a date with Lana in "Rush" shows who really rules that roost. Best Episode: Insurgence.
Lionel is a great original character, one of SV's best contributions to canon. But by the middle of the series there was writer-flail about how to deal with him. The confusion set in after the magnificent Transference. After having Clark inside him, Lionel becomes "kinda" good. Not to the extent of not having people killed (Lexmas) but good enough to offer advice to Lana Lang. (Sacred.) The Jor-El possession supposedly makes him even less bad, with Martian Manhunter around to vouch for him...but he never actually stops doing bad stuff, which leads to the puzzling schism of S6 in which Lionel can have cozy confabs with Clark (and Thanksgiving dinner!) but still indulge in blackmail & death threats when the plot requires it. (Promise.) It was like the writers' room was battling over Lionel's soul.
Best: Season 3
Hard to pick a best because Glover is so consistently awesome. But S3 is Lionel-unleashed. We find out he killed his parents, see him experiment on his son's brain, order the mass murder of a lab full of people and scariest of all, shave his head to an opera soundtrack! He's expanded from terrifying Lex to scaring the heck out of everyone. (As a side note, when other fans were debating the Chlois theory at TWoP, there was one lonely fan who kept arguing for the Lexionel theory - that Lionel would turn out to be the real Lex. The heart-transfer finale kinda proved him right & I hope he took some satisfaction from that!) Best episode: Asylum
Worst: Season 6
KK does a stand-up job with the material and once cited this as her favourite season. But this was a hard slog for any actor. Not only did they air episodes out of order, disrupting Lana's arc, Lana as written in S6 totally plays against Kristin's strengths. We're supposed to swallow that Lana goes immediately from being madly in love with Clark to blindly in love with Lex. The show tries to posit that Lana has no choice but to go live with Lex: I mean, Zod wrecked her dorm! (Er, both Lois & Chloe managed to house themselves just fine.) So she has to move in with him and start having sex with him and no, it's not just fun casual sex because Lana's a good girl. She loves Lex so she's blind to his faults and it's her intrinsic purity that stops her from listening to any advice on the subject.. except she keeps hinting to Clark that she'd ditch Lex in a heartbeat to come back to him. The show keeps hammering the "It's not Lana's fault!" soapy-martyrdom mantra when it would have been a lot more fun if Lana simply wanted Lex because he was hot, the sex was great and she was willing to overlook a few clones about the place.
Best: Season 2
Kristin has a dry wit which comes out in interviews. But she has never seemed like the "swept away by passion" type. One of the biggest failings of SV was not letting her play to type: the grounded, sensible one who cuts through other characters' silly moments. The season that came closest to achieving this was S2. I can believe Lana running a coffee shop - after all Kristin was running her own career at the same age. The scene where she gently fires Clark for being the world's least reliable waiter is perfect. S2 also lets Lana mess up a few times, in plausible ways. Looking at Clark photos of Chloe's computer? Skeevy, but I'd be tempted too. Breaking up with Whitney by video? In 2011, it would be by text message, and yes, I believe a 16 year old might do it. Barging in on her birth father? ("He called me a Nazi!") Heh. Dodging visiting sick Clark because she's afraid of her feelings? I can get that. And there are empowering moments too, particularly telling Clark she doesn't always want to rely on his rescues and learning some self defense moves herself. Best episode: Lineage.
Worst: season 9
This is an easy choice - Chloe's S9 plotline making not a whit of sense to me. She's presented as all tense and stroppy with Clark and it's never explained why. They seemed to be building to a confrontation which would have worked very well - she could have found out he'd messed with her memories and been cross about that; Clark had every reason to be carrying some resentment over the Doomsday debacle and the unasked for surveillance. Also, Chloe and Clark fight well - Tom & Allison rocking their conflict scenes in Zero & Lineage particularly. But Chloe's plotline goes nowhere and we don't get a big acting moment or any catharsis - I had to wonder if the writers dropped whatever they'd originally planned for her. (And if so, why?)
Best: season 3
The SV girls generally get judged more harshly on moral grounds than the guys do, as though SV was a virtue contest. "She's not morally upright enough to be Clark's girlfriend!" et al. I judge characters solely on their entertainment value to me which is why Lionel > Jonathan and Tess > all other Gals. So the fact that Chloe is doing some shady dealings in S3 doesn't bother me one bit because it's very entertaining to watch. Part of the enjoyment of high school Chloe is that the way she runs off the mouth and thinks she's ready to play in the grown-up pool is completely relatable. I knew that kid, who always seemed to be on first name terms with grown-ups and you half envied her and half wanted to smack her. I also like Chloe best before she knows Clark's secret. Because before she knows, she's his friend but her curiosity makes her a bit of a threat, which gave the relationship an interesting tension.
Best moments: Extinction: "He had GILLS, Clark!"; the trip to the morgue with Lex; a most excellent & believably angry quarrel between Chloe & Clark in Exile. Best episode: Truth.
Worst: season 8
Of all the wasted opportunities of SV, the writing of Oliver as an angsty murderer has to belong at the top. Oliver should have been a light-hearted character, comic relief in the show's darker moments, a sidekick for Clark, Instead, this season ends with him stabbing Clark in the back, literally, then expecting an apology from Clark for getting in his way. For me, the character never recovered from this.
Best: season 10
There was some fan-complaining about how the relationship with Chloe had "ruined" Oliver. I don't have much interest in that 'ship but it did make me wonder - what good old days are these folks remembering: Ollie being introduced as a kidnapper who condones torture? Flashbacks to his bully Excelsior days? The times he tried to kill Lex and then succeeded?
If I had to choose one episode for Oliver's best, it would be "Kandor." Oliver gets to be funny and Justin has never looked lovelier. But overall, S10 was a step up for him. He doesn't murder anyone, he mostly stopped whining at Clark and being in a relationship seemed to ground the writers conception of him. Plus, he made for one awesome showgirl!
Worst: season 5
After a great intro in Crusade, Lois spent a lot of time over the next four seasons consigned to the "C" plots of the episode, the writers slotting her in wherever there was a gap. So she doesn't usually get the juicy plotlines; she gets the "working on Jonathan's campaign" plotline and a lot of her scenes ended up deleted. I chose S5 as worst because I hardly remember her in it - it's a memory-blur of bikinis, horrible banter with Aquaman, a head-to-toe pink outfit which made her look like the pink panther...
Best: season 8
Part of the charm of Lois in S8 is Erica's palpable excitement at finally getting plotlines, some decent dialogue and a much better wardrobe. Yay, I'm off the bench! Sure, she's not in every episode but she gets to represent the light-hearted side of a a thematically dark season and ED relishes every moment. The character has also been rebooted: this a gentler, more fleshed-out Lois and at last the writers seems to know how to write her. Best episode: Bloodline
Worst: season 9
Cassidy never fails to rock her dialogue but Tess in S9 is all over the place, the same "good or evil" waffling that affected mid-period Lionel. Sleeping with Zod seemed random. It was also distressing to see Clark choke Tess twice this season.
Best: season 10
A coherent arc, a redemptive ending and a big juicy kiss from Clark Luthor - Tess had it going on in S10! Best episode: Luthor
And now my two favourites....it's hard to choose for these two because they're the reason I watched, singly & together.
Worst: Season 6 (I'm ignoring fake Lex in S8)
The biggest mistake with Lex was depriving the audience of his POV after S4. In the earlier seasons, we got to see through Lex's eyes a lot and it was a fascinating vantage point; Clark has a secret, why won't he tell me? But by S6, the writers won't let us into his head, preferring to keep Lex enigmatic. Which makes for a lot of cryptic, dead end dialogue. Lexana scenes are shown from Lana's POV and mostly concern their relationship. Meanwhile, Lex is taking huge steps offscreen - reviving 33.1, starting his cloning project but we get only glimpses of this, in between relationship drama.
The other mistake is denying Lex any victories. To be a Clark's nemesis, he needs a few checks in the "win" column, ie he wins the election, kicks Oliver's ass, Lana really falls in love with him. As victory succeeds victory, Lex would get more arrogant and more dangerous. But by S6, the show lacked the confidence to step away from the relationship drama and Lex isn't allowed even a momentary victory. Consequently, he never poses a real danger. Even the finale, he re-appears dramatically....to shake his pom-poms and urge Clark to defeat Darkseid! (From a Clex viewpoint, I'll confess to loving that.)
Best: Season 1
Michael recently said that season one was his favourite. It's hard to pick...I adore S3. But from a character standpoint, S1 might be his best. Because Lex in S1 gets shit done! He's not beaten down by Lionel yet, is still willing to take him on. Clark is the smitten one at this point, ordering Lex to drive more carefully and sulking at the arrival of Victoria. Lex chalks up a series of victories in S1, outwitting Nixon, Victoria and her father, Carrie Castle and Dominic. He's sexy, likeable and trying to be good - but he's also a bit of a menace.
Worst: season 7
Episodes like Promise, Pandora, Persuasion and Doomsday were worse to me than anything in S7. However, it is extremely low on Clark. Tom has his lowest screen time in the history of the show and Clark mostly reacts to other characters. He also gets rescued by other people a lot.
Best: Season 1
One thing I realized while watching the finale: I preferred when people didn't know Clark's secret. The first hour of the finale is one long succession of people lining up to advise and pep talk Clark. The difficulty with others knowing Clark's secret is inevitably the writers portray them as Clark's advisers. Yes, it solves the writers' issues of how to integrate other characters into Clark's life but it also makes him seem like he can't do anything without a team of Life Coaches hovering at his elbow.
Now, look back to Jitters and 15 year old Clark solving a crisis, completely on his own. He strategically uses his powers, makes a plan, saves everybody and even has sympathy for the krypto-villain. Clark in S1 trusts his own judgment and his judgment is good. He's got the confidence to disregard his parents' advice when he thinks they're wrong (Zero, Tempest), his crush on Lana doesn't prevent him from telling her she's being unfair (Hug), when he wants Chloe out of his business he tells her (Zero) and though he's bighearted enough to eventually forgive Whitney, that doesn't come right away. Contrast that to S9, where Clark isn't allowed even a momentary anger at Oliver for the knife in the back.
Another thing I realized watching the finale - perfect adherence to canon was ultimately not that important to me. As with the Justice League shows, Smallville's innovations to canon were often the best bits. There was fan-fussing that Tom wasn't shown in the suit head to toe at the end, that no one called him Superman. But to me, he was Superman right from the start, perhaps never more so when he was doing it all on his own, by instinct, sans support group.