OK, I've got or had all four and here's my opinion:
Glock 21: Blocky and definitely not a concealed carry piece. Although you didn't say this was a consideration, this isn't the guy for comfort carry. My friend's 21 was a jam-o-matic and he ended up chucking it. I sold mine too to buy the next gun, which is:
SIG 220: Fantastic gun. Best DA I've ever shot/had. My wife who has only shot a pistol one time grouped perfectly with it. Very controllable, lightweight, and carries quite well, even though it is a large frame pistol. Can't recommend it highly enough. I've introduced it to three friends and all have ended up buying one.
HK USP 45: Fairly lightweight, and very reliable. Not the prettiest pistol to look at, but real accurate and durable.
Kimber: I have the Ultra CDP and used to have the high-cap polymer. The polymer was a jam-o-matic, but the CDP is an excellent gun and the one I find myself carrying most often. Lightweight, good trigger pull, and very accurate for a small gun.
So if I could only have one, which would it be? Hard choice, but I'd have to go with the SIG 220.