Don't know if it's the best, but I'll throw
Krav Maga out there.
From the Wikipedia site:
"In Krav Maga, there are no hard-and-fast rules. It is not a sport, and there are no competitions. All the techniques focus on maximum efficiency in real-life conditions. Krav Maga generally assumes a
no quarter situation; the attacks and defenses are intended to inflict the most pain possible on the opponent in the least amount of time. Groin strikes, headbutts, and other efficient and potentially brutal attacks are emphasized."