The sub-plot with what's her name really ruined the last season for me, especially since the focus was so much on her and that the creators of the show literally said they had no idea what to do with her, had no idea why audiences hated her so much and then it struck them to make her mole/double agent.
Even though a)having a character on a show you don't know what to do with is poor writing, b)audiences hated her because her and her personal drama was soaking up way too much time and her trouble with her boyfriend or whatever was just dumb, we tune in to see Jack Bauer, dammit!, c) WHAT?! you thought making her CTU's umpteenth mole was a good idea?!
It also completely contradicted everything that came before with the character. But I guess the writers thought if you took sucky and added a new sucky, everything about that character would cease to suck.
Same thing happened in season 5 where the whole "I'm really a villain" turn was a contradiction, but people really enjoyed that one.
Plan your turncoats in advance, 24!