If I could only have one handgun... hmm...
I would first say that price doesn't matter, if I can only have one, I'm going to spend whatever I want to make sure I get exactly whta I want.
Secondly, i'm going to want verstatility, shooting different calibers for different purposes would be a plus, from hunting, to defense, to plinking, ect.
Thirdly, Reliability, so I'd probably go with a revolver, they tend to have less breakable parts, seem to be more reliable in my expirience.
Considering all things, I'd probably with the with S&W 460, 5".
can shoot 45lc, 454, and 460 for ultimate versatility, isn't TOO big (5" barrel is better than the 12 or 14).
Expensive... but I don't care, I can only get one