I like several, but I think my favorite is the Magpul MIAD grip. I sell boat-loads of them, and once you try one, you will know why.
The big thing that I like about the MIAD, as well as a few other grips, is that it sweeps up to cover the rear of the lower receiver, just above where an A-2 grip stops. In the VERY cold Minnesota climate, letting the web between your thumb and hand rest on that bare aluminum will give you frostbite in short order. The MIAD has a bunch of different panels that you can use to make the grip fit YOUR hand.