OOh - so hard to choose. I think the last series was fascinating so would probably go for that as it seemed more "grown up" in terms of issues and strong personalities (not that they were all grown up..).
Didnt like the house divided series - I liked the cast but not the glass wall rubbish.
I'd like to see a repeat of the first series - I remember the housemates as being quite innocent of the effect they had on the viewers. Every series since then has had the housemates playing up more and more to the cameras. But I still watch!