Josh Hamilton, but with him there are always a ton of "ifs." IF his head is right, IF he feels like putting out an effort, IF he's clean, IF he's not in a batshit loony mood...
On those rare days when everything fell into place with him, I've never seen a ballplayer any better. It'll always be a disappointment that those days came so rarely.
But on those good days, my gosh, he was unbelievable. (I use the past tense, because with the way his head is now, I don't know how many more of the great days we'll see.)