I am with Justin and Regulator on this,
ammo, more ammo and some more ammo.
8 mags on your vest, LBE or rig and a bandolier of 150 as a backup in a day sack.
A couple of grenades if you can get your hands on them.

Be prepared to drop everything and run, or get hit in a vehicle and get out in seconds. A grab bag of water, ammo and first aid is a winner.

As an engineer get yourself a small minestrike extraction kit, nothing special half a dozen nails, a bit of minetape and a prodder.