I'd recommend two seasons (although I can't remember which BB season they were). The first I'd recommend is S2 I think. This is the first Dr. Will season. To me, Dr. Will wrote the book on how to win Big Brother (or almost any of these types of shows for that matter). He was an expert at playing the social game while also being able to either win a competition or have his alliance win for him.
The second season is the first one with Dan. Dan, was just awesome to watch playing both sides of the house and Houdini-ing himself off the block. Edit: This was S10
I'll bet the popular consensus here will be the Kaysar-Janelle-Howie season, but I coldn't stand any of them, so for me it was not as good.
Last season was really good too, with the return of Dan.
I hated the Rachel Riley seasons, and even stopped watching the season you watched after it was apparent she was going to win.