Sorry in advance for the wall of text.
As it stands, I rank the seasons in three tiers.
Top Tier
Season 1 - Insurmountable. Despite all the years of '24' we've seen, I don't think anything quite matches the freshness and sense of innovation that radiates from Season 1. It was groundbreaking then and it still feels that way now. The first 13 episodes are arguably the strongest and most consistent in the show's entire run.
It falters in the afternoon hours ever so slightly, but these are insignificant when cast against the otherwise brilliant plotting. The Nina reveal and Teri's subsequent death were paradigm-shifting events for television thrillers and whose impact were only possible as a result of the show's unique and powerful format.
Season 8 - Go figure. Yes, I think Season 8 is, despite popular assertion, the closest the show ever came to recapturing the innovative and intimate brilliance of Season 1. It's also the closest the show ever came to truly breaking its own formula. For the first time season Season 1, a devastating terrorist attack targeting Americans or a world war is no longer the focal threat.
The nuclear fuel rods are intended to reconstitute Kamistan's nuclear program and the dirty bomb is primarily a means to lure President Hassan out into the open. The season is framed around a single concept - the peace agreement - as opposed to a series of increasingly audacious terrorist attacks like Seasons 4, 6 and 7. What's more, the last 8 episodes sees a threat against innocent lives disappear entirely. The only threat is against the peace agreement itself and the collateral damage caused by Jack's rage-fueled rampage.
Several other elements contribute to Season 8's severely understated quality. The cinematography takes cues from Stephen Hopkin's directing with greater use of sweeping skyline shots. These were largely absent from Jon Cassar's run as lead director of the series between seasons 2 and 7.
As a character, Jack displays interesting changes in perspective following his brush with death in Season 7. His sole reason for staying on board from Episode 4 onward is merely to help Renee. He concerns himself less with the 'big picture' and more with the 'little picture'.
All of these examples illustrate a season that tried very hard to take it's time building a rock solid foundation and subvert what audiences expected from the show in surprisingly subtle and clever ways. There's no question that the Dana Walsh storyline bogs the season down, but I fail to see why it gets harsher treatment than Kim's arc in Season 2.
As most agree, the last 8 episodes are nothing short of brilliant. Jack goes down a road that closely mirror's Tony's and leads to a conclusion that was as definitive as the series could have hoped for by that point in time (which LAD went on to royally squander).
Season 5 - Like Season 6, Season 5's focal threat revolves around finding and securing numerous weapons of mass destruction -- in this case, nerve gas canisters. Unlike Season 6, however, the story is framed by a string underlying plot current -- bringing those responsible for David Palmer's death to justice. This gives greater meaning to the story and a stronger purpose to the characters. It also gives the season a strong sense of cohesion.
There's little else I can say about Season 5 that hasn't been said before. The premiere was great and the finale was brilliant. Logan steps up and becomes arguably the greatest villain in the series. It deserves all the accolades it gets.
Mid-Tier
Season 4 - Season 4 lacks the thematic strength to rank alongside the very best of the show, but what it displays in spades is a thrilling plot that really captures the socio-political atmosphere in America at the time. It was the first season of the show to jump from terrorist plot to terrorist plot, and Marwan's incessant escaping was ridiculous, but it was all executed with greater finesse and convction than any other season. It was genuinely thrilling to watch it unfold.
The season is further bouyed by the constant stream of returning characters. Again, this added a sense of excitement that the show rarely, if ever, replicated.
The political storyline is virtually non-existant until Air Force One is shot down, at which time Logan makes his first appearance. In retrospect, this is probably the second biggest paradigm-shift in the show's overarching story next to Teri's death. Without Logan's rise to power, the rest of the series simply would not - and could not - have happened.
Season 3 - Similarly framed around a single idea - the Cordilla Virus - but lacking energy in its first half. The sting operation idea was a good one, and the show's foray into Mexico was somewhat interesting, but it's only when the excellent Stephen Saunders emerges that the season gathers serious momentum. From there, the show churns out some of its best string of episodes ever. Structurally, it's reminisicent of Season 8, although I find the latter to be infinitely more interesting in totality.
Perhaps the most interesting thing that Season 3 establishes that reverbrates through the rest of the series is the mirroring character arcs of Jack Bauer and Tony Almeida. Tony's act of treason in saving Michelle lays the groundwork and sets a precedent for who he becomes and what he does in Season 7.
Season 7 - A train-wreck in terms of plot, the only thing that prevents Season 7 from joining the bottom tier seasons is its exceptional character work and its strong thematic content.
Like season 4, season 7 jumps from terrorist plot to terrorist plot but the execution is haphazard at best. It lacks the finesse and conviction of season 4, perhaps as a result of the harsh criticism the show copped following season 6. It is perhaps the one season that feels least like a genuine 24 hour day.
Fortunately, the season is held together by a cast of great new characters, interesting self-conscious social commentary, Jack's arc of redemption, and Tony's descent into corruption which was first hinted at in Season 3. The character work is brilliant - Jack's exposure to the pathogen was a master stroke that allowed him to regain some of his humanity and Tony's path leads him to losing his.
Bottom Tier
Season 9 - A real disappointment after the first viewing. I had high expectations (probably my undoing). Although the season had some good moments and a few interesting ideas, I think it's a missed opportunity that completely squandered the ending of Season 8 with a derivative and deeply unsatsifying conclusion.
For me, Live Another Day is 24 running through the motions more mechanically than it ever had done previously. I think this is partly a symtpom of the compressed format. Despite the removal of 'filler', there's little room for the plot or the characters to 'breathe'. In previous seasons, filler at least allowed for more organic plot / character development and interaction.
To its credit, Mark was a great character and probably the best of the new additions. Jack's badass behavior was compelling to watch, although I'm disappointed it didn't reveal any new insights into the character. I liked that Jack's rampage in Season 8 wasn't forgotten about and that he had to face up to the consequences. I also liked how saving Heller inadverently led to Audrey's demise - a dark but well-plotted fate.
Overall, however, the season felt like a 'greatest hits' compilation rather than a fresh entry in the franchise with worthwhile things to say about the world or the characters.
Season 2 - The best thing I can say about season 2 is that there's not a lot that's bad about it. I think it's a wholly unmemorable affair with few peaks or valleys that distinquish it against the other seasons (even the less than satisfactory ones).
The obvious high point, of course, is George Mason's sacrifice. The stretch of episodes featuring Nina are also excellent. The obvious low point, of course, is Kim's entire story arc.
Overall, I think season 2 is competent but largely forgettable.
Season 6 - The ranking that will come as no surprise to anyone. Season 6 is over-blown, over-stuffed and the most overtly derivative of all the show's season.
It's biggest issue is structural in nature. The opening 4 episodes move at breakneck pace and the season more or less peaks with the detonation of the nuclear device in Valencia. It is the visceral high point of the season that it completely fails to live up to or even remotely match. From here, it had nowhere to go but downhill.
The season attempts to replicate the personal scope of the first season by introducing Jack's 'evil' family, but the end result is little better than something you would expect from a daytime soap opera.
Casting issues are also a big problem. Firstly, there are far too many principal characters (a trend that started with Season 5 which continues to this day). Secondly, there are some obvious misfires. Wayne Palmer was good fodder for his brother, David, but lacks the presence to convince anyone (the audience included) of his Presidential authority.
In fairness, Episode 17 is brief return to form. It's a clever entry that genuinely manages to shock and excite viewers. The finale is also excellent, with a great set-piece on the oil rig and an powerful ending.
Overall, season 6 was a disaster from which the show was, arguably, never able to fully recover from.