I think my favouite is Kirroughtree as it flows really well. The black is nicely varied. It also has some good technical bits to test yourself.
However I also love Dalbeattie. I'm not sure why. The rock is fantastically grippy which encourages you to try stuff you ordinarily wouldn't.
I didn't think a great deal of Mabie and Ae. They weren't bad but I didn't think they had anything special about them.
Glentress and Innerleithen are worth a trip if you have not been there. Glentress is fantastic as it has all the facilities etc (so if you've travelled you can shower etc). Innerleithen has a fantastic descent.
If you are talking a weekend then I'd ride the following:
Day 1 Kirroughtree (lunch at the Cafe) and Dalbeattie
Day 2 Glentress Red (lunch at the Hub) and Innerleithen
Your legs will feel it after but this is manageable