Glock 36. Best pistol I've ever used for carrying. It's reliable, it's light, it's small and it's a .45. I try to carry a 26 sometimes but I truly think it's heavier. I carried my KelTec PF-9 for a bit but I hate shooting it and I'm horribly inaccurate with it. Either Glock is 10 time more accurate in my hands so I guess I deal with a bit of extra bulk.
I own an M & P Shield and it is excellent. Very accurate, smooth trigger, suprisingly low recoil, very very easy to conceal ( I wear it easily on either my hip or my ankle). It is relatively newer though.
M & P .45C is combat reliable if cost is an issue. It actually has a better combat record than the Glock .40 and .45 despite their popularity
(as opposed to the Glock 17 gen3 9mm -- Glock's most reliable offering).
If price does not scare you off, H & K .45C is probably the best fighting pistol in terms of reliability. It's still fairly large though. 1911's and their variants need to be maintained more often and tuned to get the same level of combat reliability. Even though it's mmy favorite gun, unless you're willing to put a lot of time and money into it you are better off with a different gun.
All this data by the way is also the opinion of many former Tier 1 Operators/Special Operations soldiers. Reference the interview with Ken Hackathorne and Larry Vickers about the H & K .45.