I dunno if Terence Tao is as PG as people come, but that might be because I don't know enough about math; his main achievement so far seems to be a joint theorem with someone else, based on the other person's prior work, and I believe I've read that great mathematicians can go downhill as they get older. Val said it pretty well recently about great geniuses generally having a large dollop of creativity that sets them apart. My favorite example of someone in that vein is John von Neumann; I don't know of anyone more PG than him off the top of my head, as he was intensely creative and also about as maxed out as we come on processing types of abilities. The whole package.