Oh man i never thoght chosing just one would be so hard! Is it just one handgun for the rest of your life? Or is it just one handgun at a time? Like, i can only own one, but if i wanted to trade it or get rid of it and get a different one, could I?
Anyway, id probably get a glock 26, just because i carry a glock 30, but i like the 9mm round and it is readily available anywhere, and i still get 10 rounds in the standard mag, just like my g30, but also have high cap mags available. Its pretty well rust resistant, and glocks are very reliable out of the box and feed pretty much any brands of ammo.
Although i had a pretty tough time deciding between that and a kahr mk9 and a springfield stainless 1911. Both are pretty guns and reliable (the kahr after break-in) and i love the heft of metal guns. Even the g26 has shortcomings IMO where the kahr and springfield excel, but i guess the glock is just more practical for me.