The Glock 21 (.45) is my gun of choice because it's small enough to carry (technically) and it will knock down just about anything around here, but if I were to only have one pistol for carrying, it wouldn't be the 21 because it's really too big AND the bullets for a .45 are very expensive. I would probably go with a .40 or 9mm. You need to think about the cost of the bullets as even if you're a billionaire, you get to a point where you're like "Hey, this is a tremendous waste of money." Yes, you can make your own, but I haven't gotten that far yet.
Also, I'm biased as I only have a few pistols but I can tell you that you can take a Glock, drop it in the sand, kick it off a cliff, poop on it, run over it with your car....and still it will fire and you can field strip it in <10 seconds.