I would say Metallica with Cliff Burton because he probably would have never let Bob Rock produce their albums and they might still make kick ass thrash. Or at least they would have made thrash into the 90's.
I fucking hate Bob Rock.
Either that, or AIC with Layne. The new album is totally awesome, but it also makes me realize how much more amazing they were with Layne. I've been listening to almost nothing but Dirt for about the last 3 days.