Most use the Winchester white boxed 230 gr FMJs or the Remington 230 gr UMC from Wal-Mart for bulk target shooting (with good results). This ammo is mostly just fine. Occasionally, you may get a defective box or two, but really, that can happen with any ammo. Personally, I handload all my own ammo, so I rarely buy any in a store anymore.
230 gr Federal HST or HST +P is considered by many to be the top performing cartridge in 45 ACP. The other perrenial undisputed favorite is 230 gr Winchester Ranger T series RA45T and RA45TP +P. Either the Federal HST or the Winchester Ranger T will undoubtedly serve you well should you ever need them as a self-defense load, and both fit & function perfectly in your USP 45.
** STAY AWAY from Hornady, and specifically Hornady TAP cartridges in your USP 45. They have been repeatedly shown to be overlength for the chamber leade in 45 caliber HK pistols ** You've been warned.
Also, be aware that the cheaper, non-brass-cased, steel and/or aluminum cased ammo is specifically advised against by HK. Many have used these lesser grades of ammo reportedly without problems, but many, many other guys have had ongoing issues with them. Saving a very few dollars is not worthwhile nor fun if it causes failures with your pistol. You can certainly try it if you like, but be aware that they are generally much less reliable as a group. If your pistol has issues while running it, the first thing to look to will be the ammo, and not the gun.
** Edited to add: DO NOT USE ANYONE ELSE'S RELOADS IN YOUR GUN. This ESPECIALLY includes gunshow reloads. If you want to learn to do your own handloading, that is fine and dandy, and we'll help you get it worked out. But don't assume that anyone else's handloads are okay to use in your gun. The 45 caliber HK chamber leade is shorter than in many other brands of barrels, and what supposedly seems to work fine in your cousin's father-in-law's best friend's 1911 will not necessarily work fine in your USP. Keep in mind that 99.9% of all blown-apart HK handguns were caused by reloaded ammo. HK does not warrant against any damage attributable to ammo. If a big name-brand commercial ammo-maker blows apart your HK, they will necessarily assume liability for your gun and for your injuries.