I have shot the hell out of all three. My personal choice is the Glock 21 for personal reasons. The trigger is best of the three for me (maybe just because I am used to it) and the thick grip compliments my hands. If the grip of the Glock 21 is not too thick for you --find one to grope before you buy any of them-- then I'd go for the Glock. For me, they have the least felt recoil and are as accurate as I am.
If it is too thick, I'd go for the XD. They are every bit as reliable as the Glock and a bit cheaper. And all of the mall ninjas seem to like them because they are the latest and greatest
They are as accurate as the glock, too. The biggest difference between the two is the XD will be a bit cheaper and the grip is noticably thinner, which is more comfortable for most people, but not me. Because the top slide is a bit thicker, I notice that muzzle flip is a bit more on the XD than I was used to with the Glock 21, but that could just be how I grip them. You cant go wrong with either one.
Although Hk makes perhaps the sexiest firearms on the planet, alongside Sig-Sauer, I am not a fan of the USP. It is as thick as the Glock 21 but feels even less ergonomic and, as mentioned, the rail is not standard. The lights and accessories for the USP 45s are more expensive than standard accessories and they cannot be interchanged should you wanna throw on an M3 tactical light and then throw it on a quad-railed M4 down the road when you really wanna get tactical. I am sure they are probably capable of greater accuracy than the other choices, but I am not a fan of the trigger pull and I have never been able to hit squat with a USP and I've never taken the training time to learn how when I can shoot bullseyes at 20m with a Glock 21 out of the box every time...
I'd hate to suggest outside of these three, but there are some very high quality 1911 .45s with tactical rails, like the Kimber Warrior and Sig GSR to name a few. And speaking of Sig, the P220R is a railed version of the Sig P220. The military is considering the Sig P220R as the replacement for the Beretta 92 9mm in the upcoming Joint Service Small Arms Program or whatever its called which is scheduled to take place late this year or early next. It is dark earth in color (which supposedly doesnt stand out with night vision goggles), is available with threaded barrel, and is fully anodized for durability. There are also standard P220Rs with trigger options like single action only (SAO), the glock-like consistant trigger pull of DAK, or the standard double action/single action (DA/SA). These run about the same price as the Hks. ...Of course these are if you don't mind single stacks...
Just my personal opinions...
TRL