My favorite overgrip is Dunlop Black Viper Dry. It's a little thinner than the white, but more tacky. The material is dense, so it still feels substantial. There is no comfort backing, so it has very good feel. I have it over a Boris Becker leather replacement grip and two Gamma Hi-tec grips.
I'm proud to report that my hi-lite reel beats my outtakes by a wide margin.