C'mon, guys... Les is awesome, but he's not exactly your heavy metal bassist, 'though he can be. And, I suppose, we're speaking about heavy metal, right? 'Cause there are a bunch of great jazz bass players as well.
To me the answer is clear - Geezer Butler, because he is not only a Godfather (one of them) of Metal, not only that he's unbelievable with bass in studio and on stage alike. He is so extremely creative, that if you think where this man comes from (I mean the background), you just can't fuckin' believe it!