Interesting comments from all around. If I were to only be allowed one handgun ever, I'd put it through the types of situations I might want to use it for. For those who only would consider using against paper targets, I suppose it would really matter.
I'd consider for which ever countries I'd possibly live in, self-defense, home security, availability of parts/mags/ammo/etc, reliability and all sorts.
In this consideration, I'd find myself more inclined towards the military, combat style of handgun. A 1911 in 45acp would suit to the task, popular, secure for just about any purpose, easy to find parts and ammo. A SIG P22x (say 220 in 45acp or 226 in 9mm) would be a more modern, expensiver way for a reliable gun. Glock 17(and I'm not even a fan) would but a popular root to go. HK USP would be similar for a hard using combat gun.
Hard to say really, so just about any service pistol would do it for me...provided I wouldn't spend more time cleaning it than carrying it and wouldn't get a caliber like 40S&W or worse 10mm(as good as they'd be, I wouldn't want to try to find ammo if I were in say Cape Town or god forbidd Mogadishu).
Oh and I suppose I would want something extremely tough and heavy along with the mags. In desperate situations, I'd probably consider using the grip as a hammer(!) or throwing it like a boomerang(haha)