My rankings:
S4: Despite the fact that the plot makes no sense, I'd still consider this the best all-around season of the show. Everything just fires on all cylinders - the action scenes are placed outside of the standard "end of episode" format, twists and turns in every episode, Jack teaming up with Tony to do field work, lots of interesting supporting characters, an episode that focused primarily on random civilians (the nuclear football one) that worked really well, Curtis coming to the forefront, the supporting characters from S1-3 slowly worked back into the fold, that killer ending...I can't say enough great things about it. This is when 24 was must see television.
S5: Still a damn good season, but not as good as S4. The pacing and action are all top-notch, but some goofy elements start creeping in, like Tony's "death", Henderson's extreme resilence, the padded-out airport siege, the "Bluetooth council" and Jack's ridiculous stunt on the passenger plane. Still, lots of great moments in this one that pay off for long-time viewers.
S1: Still pops because of the off-kilter pacing, surprise twists, soundtrack and action scenes, which are still some of the best in the series precisely because of how grounded they feel. That said, a number of subplots are crap (Alberta Greene taking over CTU, the assassin's relationship with the campaign staffer, Kim getting taken hostage twice in one day, the horrible amnesia subplot), but overall, it still is very good.
S8: Starts out with promise, loses the plot for half a season, then comes back with some of the best episodes of the series' run. No nonsense action and plot twists at the end, Jack going balls-out dark to accomplish his goals and the status quo being shaken up majorly.
S7: Kind of ambivalent about it, due to the fact that it all blended together. A few standout moments (the White House siege, Tony's reveal/underlying loyalties), some great character moments and good supporting characters.
S3: Just the various definition of the word "meandering". Lots of subplots that go nowhere (Kyle Singer, the Claudia/Mexico connection), lots of action scenes that felt like they were phoning it in (the assault on the MI6 offices, the final fight) and some wonky characterization. However, it also has a couple of my favorite moments from the series (the shootout between CTU/Saunders' men and the airstrike in ep. 22, Chloe taking center stage and proving her value to the company, Chappelle's death).
S2: Yes, it's worse than season 3. Nobody acts normally - Kim constantly doing stupid things, white supremacists randomly showing up, Jack jumping back into his trigger-happy mode like it's no big deal, Mason going out to investigate a site on his own, etc, etc. I loved it at the time, but the Kim subplot and wasted pacing in the back half of the season just throw it off completely. I hated it.
S6: Starts out amazingly for the first four episodes (the nuke, Jack rescuing Al-Assad and Curtis' death), then goes all downhill. Retconned or made characters the inverse of who they were, weird action beats, bizarre family drama. Almost uniformly terrible all the way around.