A bit of everything.
I've learned a bit of boxing, for proper blocking and evading punches as well as giving solid ones back.
Several years worth of Han Moo Do, for quick techniques to disarm, counterattack and acquire submission holds. Naturally, as with any proper martial arts, there is also extensive practice of falling properly.
Thise combined with a bit of the school of though of 'everything is a weapon' and some Krav Maga 'dirty' fighting techniques make me a bit of a all-rounder but master of none. I mainly attempt to subdue my opponents with joint-locks or submission holds or use throws to put them down long enough to run away and gain a sufficient distance from them so they can't catch me. If it doesn't work, I can always bash their head against the nearest solid object or use some of the more painful submission/disarming techniques.
I know fully well I am physically capable of killing someone unarmed. I also know several ways to do it, besides the 'bash them in the head until it caves in' method.
But I consiously avoid those methods as long as possible. The worst I've done is dislocate a shoulder; coupled with a wrist lock, that was enough to tame the guy until police arrived. Considering he attempted to rob me and beat me up with a knife, both I and the cops thought it acceptable use of force (specially since I didn't force the dislocation until lesser means proved ineffective).