If I was going into Iraq with the soldiers (G-d help us if they need the help of a 38 year old very overweight shomer shabbos man who hasn't seen any military training since BCT when he was 19) and could bring whatever I wanted I'd pick:
Out of what I own I'd take my AR 15 and SIG 226 or CZ 75B. These aren't necessarily my first choice all else being equal, and an HBAR AR may not be the best combat choice, but resupply is an issue. The AR can use M16/M4 mags and many of its replacement parts. The pistols are my only 9mm pistols and at least I could use the Army's ammo. If I bought something new I'd get a Beretta 92 for the same reason I'd bring the AR.
If we were to supply our own ammo and replacement parts (or if the government would supply us regardless of what we brought) I'd prefer something with more long range power than the AR and more powerful than the 9mm (nothing wrong with good 9mm JHP for self defense, but if using FMJ I want a .45ACP or a .40S&W). I'd probably bring a .308 semi-auto, or a medium caliber bolt rifle. First choice would be an M1A, HK G3, CETME or FN FAL- the range of a .308 (though not quite as accurate as a bolt rifle) but with 20 round mags and faster firing than a bolt rifle for close in battle. For the pistol, I'd probably go with a full-sized 1911 (I love the platform and I love .45ACP). Second choice would be a nice .40S&W- the full-size SIG 226, or my more compact 229 being very high on the list.