Season 5 was my favorite. I think they did a great job of building up tension, foreshadowing the coming of Zod. Also liked their take on Brainiac. I enjoyed just about every episode of that entire season, which not something that typically happens for Smallville with me. But from the first episode, with the emergence of Brainiac and Depeche Mode playing the background, it was on.
I don't know if I have it in me either to rewatch the series. I would like to cherry pick and watch a few episodes in every season, but I don't think that justifies me buying the series. Though I am tempted too. The series finale left me feeling good enough about the Smallville experience. Still hard to imagine I devoted ten years to watching it all come about though.
My tenative ranking of the seasons:
1. S5 (Everything came together in this season for me and left the show in the perfect jumping off point for a movie. I wish that they had made it into a movie series than Superman Returns)
2. S1 (Solid first season that sold me on the idea that a show about Clark Kent and not Superman/Superboy could be interesting. I also liked a lot of the freaks of the week and the songs at the end of each episode. Lana was so fine and the unrequited love angle was handled well enough. Lana was like the perfect high school girl to crush on. Hot but actually nice)
3. S2 (Building on the first season, I rank this one so highly because of Christopher Reeve's appearance. He passed on the torch and it was awesome. To be honest, I don't remember much of the season beyond that now.)
4. S8 (I thought bringing Doomsday to the show was a stupid idea, but you know what, they made it work. They actually made Doomsday a viable character, not the one-shot awesome engine of destruction that DC seems capable of only using the character as. Kudos to Sam Witwer for that. Even though the finale battle was too way short after doing such a good job building up the fight, I didn't mind them getting rid of Jimmy. It was stupid how they brought in the 'real' Jimmy Olsen though.)
5. S10 (Perhaps I'm still caught in the afterglow of the series finale, but I enjoyed S10 for the most part. I think the first half of the season moved 'rapidly' to finally have Clark reveal his secret to Lois and take their relationship to the next level. I also enjoyed the return of Lionel and Kara. I thought Darkseid was a great choice for big bad though I think they mishandled him. It was still great to at least see them attempt to take him on as well as his lieutenants. The anti-vigilante stuff reminded me of Marvel's Civil War, which isn't a bad idea to steal from, though I don't feel they handled it as well as they could have. As with most Smallville seasons, it was uneven and had its share of filler episodes, but the last hour delivered.
6. S3 (What I remember about this season was Emmanuelle Vaugier and the Godfather-style wipeout orchestrated by Lionel. Both were pretty awesome events.)
7. S6 (I ranked this season so lowly mainly because I didn't see all of it. Plus it took me a long time to accept Justin Hartley as Oliver Queen. I didn't like his acting. I did like the idea of the Zone phantoms, the finale with Bizarro was good (even though I think Chloe should've died in the finale, sorry), and the Justice League episode was one of the best in the entire series run.
8. S7 (I thought this season was okay. Liked the choice for Supergirl. Was intrigued with the El family drama. Veritas angle had potential, but I didn't care for the finale. Before that I didn't like Lex's final turn. Even though it should've happened seasons ago by this point, it felt rushed to me. I was cool to the season finale, where Lex learned everything as well. However, I think Rosenbaum did a great job acting wise. One of the few season finales I didn't like.
9. S9 (I liked Absolute Justice and Metallo. I liked the idea of bringing Checkmate in. I hate to say it but Pam Grier didn't do much for me as Amanda Waller. Would've loved if they could've gotten CCH Pounder or Garcelle Beauvais-who played a Walleresque character on Human Target that year). But this season ultimately didn't work for me because I couldn't stand Callum Blue as Zod. I thought he overacted.)
10. S4 (The weakest season for me because it was unfocused and went off on this weird Lana as witch tangent. I wish the writers had their acts together and tightened up the Veritas storylines to make the Teagues more influential in the overall scheme of the show. I did like Jane Seymour and Jensen Ackles, but I felt they were misused. That being said, the series finale was one my favorites (back in the day at least). For Onyx was in this season. That was a great episode.