Who has been the smallest player ever to play the NBA?

It is Muggsy Bogues

Muggsy Bogues is the shortest player in NBA history, listed at 5'3”. He was drafted 12th overall by the Washington Bullets in 1987 and played later played for the Hornets, Warriors and Raptors.

He is only 160cm tall.

There is a list of shortest NBA players here.