I personally think Season 6 is somewhat unfairly slammed by just about everyone - granted it's not the series' finest hour, but every season of the show has strengths and weaknesses. The main problem with 6 is that the writers outdid themselves far too early - after the 2 cataclysmic events that happen in rapid succession in the 4th episode, the season very quickly runs out of steam. By the end it's all a bit of a farce, but it's by no means as painful as some make it out to be (this is 24, after all!)
And as for the rest, in my estimation:
Day 1: Very good, sagging a little just before the series-defining finale.
Day 2: Stronger season overall, hampered only by Kim's hopelessly inept and irrelevant side-story.
Day 3: Decent first half, tremendous second half, with some of the series' best episodes.
Day 4: Weak-ish season overall, which tries to overhaul the established formula, but then does some serious back-tracking halfway through and ends up exactly as before by the end. Has its moments though.
Day 5: The best season by a country mile. Consistently sensational.
Day 7: A great start slowly dissolves into a very average season.
Day 8: 2 episodes in, and a decent if unspectacular start. Hoping for the best though!
Can you tell I'm a 24 nut?