RedFive said:

Most questionable? Everything after season 4 sucked assholes..

Questionable in a political, and moral sense. There was just WAY too much fucked up shit in there.
Jack Bauer tortures a guy he caught at a suspicious place, against the (rightful) objections of an Amnesty attorney, and he just happens to know "where Marwan" is.

That was probably the most fucked-up shit they've done. and as far as I remember, it's never gotten this bad again.
5 and 7 were awesome, I kinda enjoyed 6, and 8 had its cool moments (as well as some of the lamest plot devices ever).

But yea, haven't watched that stuff in a while...