The HK 416. I'd like to use the 6.8mm variant, but the ammo would be too rare. Even chambered for 5.56mm, it's still far more reliable than the regular Colt M4 in every way. (You can completely submerge the gun in water and flood the firing chamber, and you'll still be able to fire.) Plus, it has the added bonus of being able to use the same magazines as regular M4s, so on the rare but still plausible chance that I find one, I can grab some spare ammo.
If I can't use a gun, I'd have to go with the good 'ol 7 iron.