I find it hard to compare because it's been 8 years since I saw my first series of BB, and it's hard to realistically know how it stacks up against recent series, but my rough ranking is:
BB10 (Loved the winner, loved the drama, characters to root for and against)
BB8 (Most people this year were unlikeable apart from the goddess that is Jen, but it was very entertaining, and I loved the final two)
BB6 (Great cast, but I never got the Janelle adoration, and didn't watch it in real time, so lost some of the suspense I suppose, knowing how it was going to play out)
BB11 (Lots of unpleasant characters like Ronnie, Jessie, Lydia and Natalie, but all were triumphed by a lovely winner. Although completely rigged with the coup d'etat, and some great characters such as Laura and Casey lost criminally early)
BB4 (This was my first season and I loved it, but I can't rate it much higher than this because I can't remember much of it...would have been better had Alison won, her making the final 2 was one of the best bullet-dodging plays that BB has ever seen)
BB7 (Liked the series because of Danielle and Will amongst others, but disliked the early dominance of the BB6 players, especially as it resulted in the loss of some potentially more interesting players...I'd have much rather have had Alison or Diane hang around longer than loudmouth Howie, for example. Underwhelming final 2.)
BB9 (Too many people, too many morons)
BB12 (Just awful gameplay; easiest victory ever for any member of the Brigade, great characters in Brenchal and Britney, but everyone else was just completely vapid, like Kathy and Lane, or just really annoying, like Enzo and Matt. If they'd used their brains they could have stopped the Brigade, but Hayden walked to an easy win.)
Haven't seen BB1, BB2, BB3, BB5
BB13 is currently hovering in the middle at this point.