Annihilus would beat BRB or at least match him. He did extremely well against Thor.
Also, Thor only really holds back against humans/mortals/friends. I don't see a reason why he would hold back against Annihilus, especially given the context in the story where they fought (wherein Annihilus fed on the lifeforce of several Asgardians before he was discovered by Thor).
6/10 or 5/10 going to Annihius here.