Since there has been mention of hunting as well as defense and I don't own a magnum right now. I would have to say the gun I do have that I would turn to first is my CZ-52.
Using hp ammo would allow me to hunt and ball ammo would allow me to to shoot through most common items including class 3 body armor.
Actually, I do only have one handgun. I was limited financially to just one, and I think I have the best. A Glock 19. I've not gotten to practice in a long while however, but I think I'll be getting back to the range for a session very soon.
Soundslikejosh has provided some good wisdom. I've personally had pretty much every type of handgun at this point. I'm a 1911 45acp fan for personal defense, however, if I were limited to only one handgun it would probably be a 4 or 6 inch 357 mag S&W 686. The 4" is as close to a one gun for all purposes as possible. Shoot 38 specials for practice and plinking...357 mag for defense and hunting.
One gun, forever and ever? Ordinarily, I'd join the Glocklove bandwagon, but one gun forever... 4" .357 SA/DA revolver. Fewer parts to go sour on you, coupled with the fact that the learning curve on a revolver is non-existant compared to a auto puts it on top in my book. I only have to say point it that way and start pulling the trigger, and anyone can operate it.