Digi, skip all of season 4 except for the Chinese Government related eps, and you're good to go. Even as a dedicated fan, i'm doing everything in my power to erase the utter crap that was season 4. ugh.
I can ditto that right up to the final word. For me it was season 3... I try not to count season 1 when comparing them, because it was the first, the beginning. It stands above the rest regardless of how good they get.
I though S2 was great, nay, incredible, what with the Nuclear focus and all, but still not quiteas good as S3. S3 involved a lot more emotional and ethical focus for Bauer than 4 or even 5 have so far. And let's face it, Jack was practically transparent in S4
For me, in order it's (S1)-S3-S5-S2-SX (X = unknown, most likely redundant).