Legally transferrable Reissing SMG. They are much, much cheaper than their Thompson equivalents, and a lighter and more modern gun overall.
For flat-out best historical collector gun, I'd go with a BAR, but it'd probably run as much as a full auto Thompson these days
. Since you say "only one" I assume you are not interested in becoming a milsurp-buff, in which case I would argue the Garand is the most practical of the choices (the carbine is nice, but can't do as many mundane tasks as a 30-06/308). The 1911 is more practical yet than the Garand since it can be carried, but the originals still remaining really don't deserve being soaked in belly-sweat all day considering how much they go for.
If you are interested in WWII-
era guns in general (immediate post-war and modern reproductions included), I would heartily recommend the FN49, BM59, SVT40, K98, PPSH41, in that order, and the Hi-Power, Webley/Enfield, Luger pistol, and S&W 1917 in that order if a pistol is more your speed.
"Only one." Ha!
TCB