I’d rank Season 2 highest - interesting plot in general, the Kim subplot was so ridiculous that it served as comic relief, and all the side plots held my interest - Jack’s love interest and her family, Sherry’s dealings with the baddies in order to bring down the President, the few episodes with Nina, and in particular all the crazy politicking surrounding the Cabinet’s deposing of the President and the conflict between the loyal Lynn Kresge and the (surprisingly) disloyal Mike Novick. Just generally thrilling the whole way through.
I’d probably rank either Season 3 or Season 5 in second place. Unlike apparently everyone else, I thought Logan made a pretty compelling villain, and I enjoyed the general plottery by the various factions at CTU. Plus, while the plot was twisty, it still made pretty good sense overall, and doesn’t fall apart in hindsight. Season 3 had a great ending, but I thought the act I, with the Salazars, was pretty slow some of the time. On the other hand, it involved Nina and Sherry, and I love them both. They’re probably the biggest thing I miss from recent seasons - both of them were so compellingly wicked, and Sherry was an awesome foil for David, taking full advantage of his decency to exploit him at every opportunity. Plus it had Ryan Chapelle dying. I first started watching 24 when I was living with my dad and happened to wander by while he was watching. I got ensnared when I saw Jack actually made the decision to kill his boss. Was he really gonna turn a gun on a CTU agent? And he did. I was very impressed by the show’s willingness to explore some moral ambiguity on the part of the good guys. (Also, Stephen Saunders was one hot supervillain.) By the time I finished watching what I had missed, I couldn’t wait to get to the end of Season 3 to see Ryan Chapelle get his ass killed.
Season 1 is next; I don’t know - it didn’t grab me. The Kim and Terry plot bored me to tears, and the main Jack plot was confusing and scattered. I didn’t particularly care for the Serbian family as villains - they just weren’t very interesting. Though I did enjoy Nina; I found the actress who played her (can’t remember the name) compelling as both good guy and baddy. Even knowing what was gonna happen, it was still a disappointment when she was revealed as evil.
Season 4 is definitely the worst. I can barely remember what happened, the plot was so scattered. Great characters, particularly Shohreh Aghdashloo and family. But the storyline was such a confused mess that I didn’t enjoy it at all.
So I think Season 5 is about as good as the show has been all along. Except for Season 4, I think the show has remained fairly consistently good in quality over all five seasons. I didn’t find Season 5 any less believable than any of the other seasons - none of them are really plausible in the end, but that’s not really what I watch for, and Season 5 didn’t stretch my suspension of disbelief particularly far.