FairWarning has some real good arguments, . . . Plus, . . .
IF, . . . I could only have one, . . . it would have to be the one that I know the most about, . . . am most comfortable shooting it, . . . the one that I like to shoot the most, . . . and would have to be one that I can expect to be able to find ammo for just about any place I go.
The old .45 ACP fills "my" requirements, . . . YMMV.
May God bless,
Dwight