I strongly believe that Fischer could have been greater than he was. The issue is that once he was top, where else could he really go? There was no one on par, and much worse for him, he didn't *see* anyone as a threat.
If Magnus and Fischer lived at the same time, either era, we would probably have one of them break the 2900 barrier, pushing each other further, albeit, I think Fischer might burn out in frustration overtime. Still, that would not mean he was weaker than Magnus, but merely, he would grow impatient with his attempts to place Magnus substantially behind him.
The exact interaction between them would be more interesting than who wins a single match. I would actually hope Fischer would lose at first so he stays interested, and then catch up to him so he has a goal.
If only we could see that possible world. D:
Carlsen's game as mentioned by pfren.
http://www.chessbase.com/newsdetail.asp?newsid=8257