If by best, you mean the one with the longest reliabilty record it would probably be either HK's USP or Glock's model 19. Both have a long history of reliable operation in all kinds of nasty conditions.
Having said that, there are a lot of newer guns that may turn out to be just a tough...S&W's M&P....Springfield's XP....are two that come to mind.
Having said that, there are a lot of newer guns that may turn out to be just a tough...S&W's M&P....Springfield's XP....are two that come to mind.