I think I've watched every series about 5 times, possibly more for season 6.
It's funny how it's evolved. Ru was so much friendlier at the start, now watching him try to care is quite amusing. The queens are so much more polished too in later series. Some of the earlier queens were rough as arseholes but were beyond talented (Sharon Needles). I like watching how the make up has changed too. Think about Aja in her series then when she came back for All Stars, she had improved so much.
My favourite queens are interchangeable, but Bianca Del Rio is my hero, I love Jynxx, Alyssa Edwards, Bob the Drag Queen, Monet Exchange (best lipsync ever to Nikki Minaj) and Kim Chi. There's also a few that I wouldn't get tired of slapping!