I'd prob grab my glock 17 (my bedside gun) because its dead reliable I have a shitload of ammo and spare mags for it. Plus it already has an extended slide release, trijicon nite sites, tungsten guide rod, 3.5 trigger and an insight M3 on it. That and other than .22 it would be the easiest to find ammo for.
http://i89.photobucket.com/albums/k204/stangsn95gt/9b87aa0f.jpg