Depends upon what the necessity was. If it's truly a "one-gun for everything situation", and that includes SD, hunting, etc., then it gets interesting.
The G19 just flat works for most things, but it ain't a hunting gun. If I could get away with the size (i.e., carry/clothing compliments in the area permit), a G20 or G21 covers all bases and well.
Sentimentality wise, there's a certain Colt 1911 that would get the nod right now if I could only really have one, and I'd get another barrel and spring set up to run .45Super in it for field carry.
If a revolver, the 4" K-frame in .357 (19, 65/66) does just about all that could be asked of a revolver for "doing it all". A 4" GP100 would be another consideration.
America needs to understand that our troops are not 'disposable'. Each represents a family; Fathers, Mothers, Sons, Daughters, Cousins, Uncles, Aunts... Our Citizens are our most valuable treasure; we waste far too many.
