There was an interesting topic the other day on Mos Eisley asking who is the best living game (presumably RPG) designer.
That's a tough one. I would probably say someone, change my mind 500 times, and accidentally leave out/forget someone brilliant.
Some of the interesting names brought up in that discussion, listed here only as food for thought:
-S. John Ross
Of course, I had to add one that was an egregious oversight: Marc W. Miller.
This is the sort of thing that could be debated all day and night, with no clear resolution.