CONGRATS! Welcome, sir, welcome indeed and thank you for your service!
What gun? My friend, in THIS case, it can be only one, and that would be an All-American 1911A1, preferably a Colt, the 1911-2011 commemorative version might be perfect. That was the fighting Leathernecks' pistol for many, many years, and would top it off just right, IMHO.
Once again, congrats, sir, and welcome.