Well, everyone and their grandmother has chimed in, so why shouldn't I?
I did two tours in Iraq with the 173rd Airborne and the 1/509th Airborne, one in a relatively rural setting and one in Baghdad.
The most useful knife I had (and for the life of me I can't remember the brand name) had a small, one inch blade, give or take. It was a folder with a clip that I kept attached to my belt loop. It was always at hand, and 99% of the time was all the knife I needed.
I tried a ton of different blades and tools over the course of my time in that country, depending on whether or not I was doing a mounted or dismounted patrol.
For fixed blades I recommend either a glock field knife or a seal pup, for the afore mentioned reasons. Fighting with a knife is just stupid when you have a rifle. If your rifle is out of ammo, retreat. We're American's for Pete's sake, and bullets are cheap, go get more.
If possible, wear all your gear on your chest, as stuff hanging off of your sides tends to snag on things when you are trying to get in or out of a HUMVEE or door in a hurry. That leg rig looks cool, but you feel like an ass when you try and exit your humvee, it snags, and you end up face first in the mud (that anecdote is from personal experience).
If you have the luxury of mounted patrols, get a bag full of hand tools to include a small sledge hammer, a wrecking bar and some screw drivers, then throw it in the truck. You'd be AMAZED how useful little things like that are during day to day operations, and equally amazed at how few people actually have them. Yeah, the hallagan tool sure looks cool, but it weighs about 10lbs and isn't fun to carry.
Tomahawks look cool, are a traditional American weapon (more or less) and don't weigh a lot (relatively speaking). Get a good, solid one like a VTAC, or something from Bear Mountain, and you may even find a use for it other than scaring Iraqi children and showing it off to Fobbits.
If you really want a big "LOOK AT THE SIZE OF MY PHALLUS" blade that you might get some use out of in a rural setting (I used mine for building hide sites and cutting away vegetation to look for weapons caches) look into a Woodsman's Pal. The overall opinion of them on this site is pretty low, but they look cool and mine did it's job.
And finally, try not to wear so much gear that you look like the mannequin at CIF. One folder, one fixed blade, and a multi tool is all you need. Don't look like the 5'3" girl guarding the phone trailer at Camp Victory. She had a pressed uniform and, I kid you not, 5 knives on her that I could count. I was tempted to hold her upside down and shake her, so I could give what fell out to the Iraqi cook at our little base of operations so that when we got a goat for him to cook he could actually butcher the thing.
Oh, and the Artillery 1SG with the sword (yes, an actual sword) strapped on his back and the razor blades sewn under his patches looked like an idiot too.
Well, that's my long winded reply to your post filled with opinion you didn't ask for on subjects you probably don't care about. The funny bit is that since I'm on page two or so of this thread, you probably wont even read this. That's okay, however, because if I save just one person from looking like a jackass with what appears to be a bladed agricultural implement strapped to their leg, then it's worth it.
As always, this is just my opinion, and your mileage may vary.