I assume you are asking for the "You have to leave and only pack one gun case" type of scenario. The answer to a question like "You are only allowed by the government to own one" would get a string of fight'n words I would be banned for posting here.
Anyways, back to the OP's question...
Does a lower count?
I'd pick an AR lower with as many uppers in various calibres possible to put on it. The adaptability and range of use is just great. Five uppers with the one lower would be perfect: a target and gopher .22LR, a short 5.56, a DMR style 5.56, a short brush hunting dedicated 300 Blackout and a deer hunting dedicated 7.62 x 39 (assuming they all fit on one lower, I forget about 7.62 x 39 on a .223 lower: brain fart).