No-brainer as far as I am concerned; the 357 revolver shooting 38spl. Unbreakable, totally reliable and deadly.
I have several semi-autos and DAs, and carried a 1911 in Vietnam. I wish I had my 7 shot Taurus Tracker 357mag revolver in the Nam instead. With hard sights I can hit a saucer at 50 feet 10 out of 10 times shooting 38s and it feels like an empty plastic water pistol when empty. I can't do the same with a laser sight on my 1911, even though I qualified as an expert on it in the Army.
Bigger loads are either too loud or have too much recoil (or the gun is too heavy for you to want to carry it) to be consistently accurate and rarely do you have a chance to find your ear protection in a self-defense or combat situation.
The only small load I like is the 22 magnum, but the selection of pistols for it is very limited. Ruger's new 32 magnum may prove to be a winner, but not in a 2 lb. GP101 revolver.