Season 6 is hands-down the best season of Big Brother, ever. Shit hit the fan just about every week, and there were great personalities in the house. I remember reading every feed update that year, rooting for the "good guys."
Season 8 was setting up to be really good, until the entire America's player crap ruined the season for me. The entire Dustin eviction saved/ruined the season, at the same time. It broke up a developing alliance of annoying people (Dust-bin, Jameka, Amber, Jessica, Eric), but also fixed the game in the other direction (not to mention Zach was an utter dumbass).
Watch Big Brother All-Stars after season 6. You don't need to watch the previous seasons to understand some of the other all-stars, just read on Wikipedia how the previous seasons went.