Of the guns on the list, I'd go with the M-16 family. I'm a younger guy, veteran of the ongoing conflict, and that is what I trained with. Not new by any standard, coming up on 50 years of service, but tried and true, fairly reliable and it doesn't seem to be going away any time soon.
2nd place on the list would be the 1911. Seems to be the one design most often copied, but purely American. I would have rather carried a 1911 over the Berreta M9.
+1 for the Thompson. Quintessential firearm of the prohibition era, used by G-men and Gangster alike, continued on to dominate the battlefields of WWII.
But the M-16 has my vote. M-4 just being a variant of the M-16 doesn't need to be on the list, imo.