The Walther PPK, of course. It's
the under-the-pillow weapon.
Sure, it's low on stopping power, but the element of surprise when your would-be assassin starts taking fire from his formerly sleeping mark more than makes up for it.
Also, it's small and discreet enough that it won't jab into the back of your head from its hiding spot and keep you up at night.
Anything that's good enough for James Bond is good enough for me.