I don't know if I can get a top 20, but what I do like:
The Offspring - Been listening to thier new album a lot lately. Very good comeback.
Credence Clearwater Revival
Guns N' Roses
System of a Down (And I've heard a song from the spin off group Scars on Broadway which I liked)
I don't know, I'm really getting into the hard rock alternative stuff that you'd here on Octane or The Sqizz if you have Sirius or XM (respectively). Ranking current bands is tough since it depends on the music. For example, I've heard some of Saliva's stuff as well as stuff from Trapt, but I don't know the longevity of those bands nor have I heard more stuff from them. I also listen to country but in terms of bands, there aren't that many. Mostly of them are solo artists.
I'm looking to fill my Itunes library with more hard rock (Too much pop and country and indie artists that I'm starting not to like such as a Seattle Group called Long Winters) stuff so if anyone has any suggestions, I'm listening.