I've just re-watched (or in the case of season 8, watched) all seasons within the last year, so they're all relatively fresh in my mind. Not that that helps; I've changed my mind countless times whilst coming up with the order, with the top 5 all in with a shout for the no.1 spot. Here goes:
[SPOILERS THROUGHOUT]
Season 8: The brilliant second half more then makes up for the rather lacklustre (but not without its moments) first half. Hassan's death, the EMP, Jack's revenge rampage, Logan, and Renee's near-execution while Jack listens helplessly are all highlights. A fitting send-off to an amazing series. Having said that, WHERE WAS AARON PIERCE?!
Season 3: Firstly, Chappelle's death is one of the greatest tv scenes of all time ("the only friends I have are the people I work with"... awww, you go ahead and believe that Ryan). A couple of excellent twists with the old mole storyline; first finding out that "bad-guy" Gael was in fact working for CTU, and then finding out that he wasn't a bad-guy at all. Also loved the whole hotel virus storyline, Nina's death, and Tony and Michelle. As already mentioned in other posts, season 3 also saw the introduction of classic character Chloe O'Brian (although the baby storyline is a bit silly), and one of my personal faves, Chase. The political storyline is pretty poor untill untill Sherry shows up though.
Season 5: An exhilarating season throughout. The Logan's give us the best political story since season 2, Henderson's death is one of the best bad-guy deaths in the series (along with Victor Drazen, Nina, and Dana), and Edgar's death just may be the most heartbreaking and emotional moment ive ever witnessed on tv. Shame about Tony's "death" and the fact that Palmer and Michelle were robbed of the deaths that their characters deserved in favour of a shocking opening to the season (although I suppose I'll let that slide as it was necessary for bringing Jack into the story).
Season 7: Some great new characters (Renee, Pres Taylor), Tony's return, and some truly classic 24 moments (the white house siege, Renee being buried alive, and Tony killing Larry) made this season well worth the 2 year wait. Even the mole story was alright.
Season 2: Kim's ridiculous sequence of events prevents this otherwise amazing season from challenging for the top spot. This season had without a doubt the best political storyline in my opinion, with both of the Palmer's, Mike Novick, and Lynne Kresge all on top form, as well as the first in a long line of shady vice-presidents in Charles Widmore Jim Prescott. The Marie Warner twist is possibly the best major twist that 24 has ever done; I for one was completely fooled. The episode with the nuke explosion is in my top 3 episodes of 24, bringing an end to a brilliant season for George Mason.
Season 1: An absolute game-changer that can more than stand the test of time. The first half is brilliant, particularly the Alan York twist (the moment I knew 24 was going to be something special) and the very tense episodes where Gaines is controlling Jack via earpiece. The second half has more than its fair share of dodgy storylines (Teri's amnesia, anyone?), but it ended with the best finale that 24 has ever done (Jack's rampage, Palmer's ownage of Sherry, and of course Teri's death... stunning).
Season 4: Consistently entertaining throughout, but never quite reaches true 24 greatness (with the exception of Paul Raines' death - powerful stuff). The fact that I can't think of much more to write pretty much sums up this solid but safe season.
Season 6: Better than what most people think in my opinion, but still the lowest on my list. The first 4 episodes are spectacular, and a solid pace is sustained for much of the first two thirds of the season, but then the whole circuit-board/Audrey storyline turns up. The fact that Jacks family were in on it (as well as events of season 5) was also a bit daft, although it did provide some good scenes, such as Jackcomforting his brother whilst interrogating him. The CTU love-tangle was a bit ridiculous as well (Morris-Chloe-Mylo-Nadia-Doyle... all they needed was for Morris to make a drunken pass at Doyle and it would've been a full on love-pentagon, with a gay kiss thrown in for good measure).
Long live Jack Bauer and 24! 