All in the title!
I'd go for BB7. Sure it was OK, and produced 2 of my top 10 housemates (Nikki and Asyeline

) but overall I think it was produced averagly, had terrible tasks and twists and I really don't understand how it became so popular, some parts were like watching paint dry lol
For the most under-rated series I'll go for BB6. It was the funniest series and was cleverly produced with strong, game playing and unusual housemates. Some new additions like secret tasks were brought in, and despite being 'Evil' for a second time, the series managed to pull of a fresh and new approach.