The Glock 19 and Sig P239 are the easiest pistols for a newbie to operate, since both lack an externally operated safety. If either one is loaded, the user only needs to pull the trigger to touch off a round.
The only argument against the P239 is that it's too small for a first pistol. It's designed primarily as a concealed carry piece, and not much else. It has an alloy frame and a lightweight slide, and it kicks like a mule. My P239 is extremely concealable, but at the range, I can only get about 100 rounds through it before I tire of the punishment.
The Sig P229 is a better choice as a first 9mm pistol. Sigs have a very simple manual of arms: DA/SA with a decocker. With the hammer down, a Sig is a very safe pisol, and the 10 lb DA pull helps prevents NDs. If you need to lower the hammer on a live round, simply press down on the decocker (do NOT try and lower it manually!).
My Sigs (P228/239) have more perceived muzzle flip than my Glock, due to the higher bore axis. However, I shoot very tight groups out of both.
The Glock 19 is a great pistol (it was my first, actually), but it's intolerant of sloppy gunhandling. If you put your finger on the trigger of a Glock, you must be prepared to fire a round. The trigger pull is only 5.5 lbs. A moment of carelessness can have massive consequences.
On the positive side, the trigger pull is always consistent, and one does not have to transition from a heavy DA to a light SA pull. Glocks require very little maintenance, and have a large and vocal following. Even better, once you know how to shoot a 9mm Glock, you can operate the 40 cal, 10mm, 357SIG, and 45ACP models as well (actually, you can do this with Sigs, too)
IMO, the HK USP is a little too much for the first-time shooter. It's DA/SA, but it can be carried hammer down, safety off (SIG style); hammer down, safety engaged (Beretta 92 style); or cocked and locked (1911-style). Too many options in a self-defense scenario will get someone killed right quick.
I don't like the placement of the safety/decocker, especially with my grip. My thumb rests right *below* the lever, and it's going to get chewed up at the range.