One gun not the coolest gun but the one gun I'd have to say would be a .22lr. Can carry several thousand rounds easily, and with shot placement hunt almost all game. Still a capable round for defense maybe not a Rambo style defense but still I know I wouldn't want to get shot with a .22
and no matter where you go across the country in any store or shop thousands more rounds will be available unlike the 5.56 or .223 rounds.
Not the coolest but arguably the most practical