I'd do a poll, but there are too many seasons. Today, I just finished season 8 for the first time. I originally started watching 24 in early 2011, I think, going through several episodes, and then taking weeks off at a time. Because of the large number of episodes and length of them, I forgot the premise of a lot of the seasons.
After going through Wikipedia and getting a refresher, I'd have to say season 3 is probably my favorite, just because I liked the most characters in that season. David Palmer was my favorite president in the series, and I really liked Michelle and Tony as well. I feel like after season 4ish, the show got less interesting, mostly due to its predictability.