Well, since you said...
one caliber instead of one cartridge, I'l go .452. That allows revolvers in .45acp, .45LC, .45AR and a variety of 1911s and other semi-autos in .45acp...I could even sneak .45GAP in there if I were so inclined.
If I were limited to one cartridge, it would be .357--J/K/L/N frames, and a Connan 1911-ish semi-auto.