For my aviators, I bought a bunch of different knives and rotated them around among the guys. The Wave survived and was the favorite, so I ordered and issued them. That was a decade ago, but the idea is something versatile and not too large, a good multi-tool. For infantry, you might get unwelcome attention for a personal sheath-knife. You'll be most in need of a tool to poke, prod, scrape, open containers, tighten fasteners, repair gear. Bring a good sharpener and make sure you're comfortable with it. If you plan to carry it in a sheath, the nylon ones don't hold up as well as leather, and the velcro doesn't last.
If you're going with just a knife, my Kershaw Leek G10 is my pick. Very subtle, and the thing just doesn't dull! That's a plus when you're hacking open boxes of rations, ammo, etc and don't want the hazard of a dull knife. You can find them for under $70 online. I wouldn't bring a serrated blade alone unless you're very good at sharpening serrated. I brought serrated and smooth blades to survival school, the instructors and I compared them on every task, we all preferred a sharp smooth blade over serrated.