I read somewhere that they speculate the most effective close quarter weapons against medieval armor were probably things like hammers and spiked weapons. Supposedly the English armies would pull or hook French knights off their horses, swarm them on the ground, then either beat them to death with hammers or drive daggers and spikes into weak points.

The stiletto...

"... became a favorite of 15th century knights, strong enough to pierce mail, agile enough to find it's way between plate armor, and thin enough to slip through the eye slits of an enemy's helmet ."