The middle finger. I keep my index on my left and the ring on the right.
It's weird though, I have two grips. Sometimes, I have my index on the left button and the middle finger on the right. The middle finger switches to the mousewheel when necessary. I know that it's more efficient to have three in an index-middle-ring setup, but for some reason I just don't do it all the time. In fact, even when I do have all three fingers on the mouse, I still click MOUSE2 with the index finger.
I know for sure though that it'd be more efficient to have MOUSE3 be the microphone button, but when I play games, it's always the two-finger setup.