I prefer the HK USP .45 in Variant 1 above all else. If I couldn't carry it cocked and locked, it probably wouldn't see anything other than range time. GLOCKs, SIGs, and many others are just as good as the HK, but it is all personal preference.
I chose the HK due to the ability to carry C&L, the extreme durability and reliability of the pistol, and the great accuracy it gives.
The DA trigger on my USP doesn't really suck, but it is long and heavy. If you get a variant with a manual safety, you can keep it in SA mode like the 1911. SA pull is very nice and light.
Durability and reliability are two things HK is famous for. This gun is built like a tank and will handle any .45 load you feed it. The only other pistol I found that is as strong as the HK is the Ruger .45. You can also beat the crap out of it just like a GLOCK and it will still work fine. The HK is as reliable as anything else out there. I can get mine to cycle empty .45 brass, which is not a practical test, but it shows how the gun is not very ammo sensitive.
The HK USP is just as accurate as any other pistol including SIG-Sauers, which are noted for their great accuracy. The HKs with the O-ring barrels are said to be even more accurate, although I don't know for sure. I don't know anyone who can outshoot their pistols, so this trait is probably not too important when choosing a combat pistol. The GLOCK, SIG, HK, Beretta, etc. are all more accurate than most people will ever be.