Whatever the unit carried, so the armorer could support maintenance; parts, magazines and ammo would be in stock; and interchangeability would exist between unit members in a crisis.
Can we assume - since we're clearly fantasizing anyway - that everyone will be issued the same thing, or maintenance doesn't matter.
I'm not sure what direction I'd go. Something that would compliment the m4 seems like it would be a good idea, but I'm not sure what that is. While the m4 isn't perfect, I'm not sure what weakness it has that could be compensated for with a pistol.
Maybe like a S&W .500 in case I had to make a big hole in something?
Or maybe a Glock 18c in case my rifle was down but I still wanted to make a lot of little holes?