Regulus Black is almost certain to me to be R.A.B. Sirius said that "Regulus found out how much Voldemort was willing to do to gain power." To me this means that he was ready to mutilate his soul to become immortal. Also if you remember Harry and Dumbledore could not put their hand through the basin of potion. At the end the death eaters made a barrier so that only a person with a dark mark could pass through and go up the stairs. I believe this very same barrier was put around the basin and Regulus being a former death eater was obviously branded with the dark mark and would not have to drink the potion in order to put his hand through the tank. One last thing is he addresses Voldemort as the Dark Lord. Only death eaters or former death eaters like Snape or Reggulus call him that.
These are three very strong reasons why he must be R.A.B. I haven't found anyone else who could be R.A.B.
"I face death in the hope that when you meet your match, you will be mortal once more." ... Reggulus was killed by either Voldemort or one of his death eaters. Most likely a death eater because Voldemort didn't know that Reggulus knew about the Horcruxes because the locket was still there!! He knew he would be killed for turning against Voldemort.
I'm almost convinced it's him though if anyone can come up with someone better it would be greatly appreciated.
darklordsmostfaithful