You want to throw punches? You want to knock guys out? It doesn't matter. Boxing, Kickboxing, Muay Thai, MMA, whatever. Doesn't matter. Any of those disciplines that appear straightforward are straightforward enough for your purposes. You'll be able to lay out any punk you meet outside a bar, as per the Kimbo video posted above.
But my honest suggestion to you (in order of relativity (sort of), Japanese Jujitsu, Judo, Hapkido, even Tai Chi Chuan or Aikido, under a very good instructor, of course. Or any style of kung fu or karate that work closely with Chin Na or Taijutsu (respectively). Arts that focus on grappling/throwing/joint locking, and that keep you off the ground, are the bee's knees for self defense, in my mind.
Hope that helped a bit.