I actually really liked season 7 because, even though I lot of our favorite characters were missing, killed, or warped beyind recognition (like that stuff doesn't happen in every season anyway XD), I really, really liked the bio-weapon threat, and the President's plot was interesting, which it isn't always.
Season 2 for sure! Why? The nukes! And start of Michelle and Tony's relationship! And I love George Mason and his contagion. Plus the Warners are so great, best terrorist storyline ever. Plus there's David Palmer. And a Kim storyline. Plus it's not like everyone isn't in "huge danger" the whole time. Enough reasons? :)
My favorites are 1,5, and 2. But of those three one is my favorite. It had the best storyline, I love the introduction to Jack's character, and I also love the Palmer subplot. The final episode is my favorite episode of television.