I hate these "if you could only have one" questions. If I could only have one, I wouldn't have the dozens I currently own....
Making it a gun you have owned, or currently own is an interesting twist, so I'll play this time.
If I could only keep one gun from my collections, it would be a tough call. For overall maximum utility, I'd have to choose my S&W Model 28 6" .357 mag. Powerful enough for hunting, versatile enough for small game (with .38s), quite adequate for defense (.357Mag, after all) and while on the large side for a handgun, not so big that it could not be carried, or concealed if needed.
On the other hand, I think the final gun I would just HAVE to keep, would be my Grandfather's Ithaca Flues model 12ga. Its been in the family for over 100 years, and still works the way it did when he first bought it in 1909. Works for hunting and home defense, just not very well for concealed carry.
