Regulus Black is odds on favorite for the moment (Sirius' Brother), at R.A.B. but I'm sure by the time the next book is out there will be dozens of theories.

I really do advise going back and re-reading the books before the last one comes out. It's a long-playing mystery which JK Rowling planned in great detail. I don't think she'll be handing us any red herrings at this point.

And now I'm going to back to my life... ;-D