Left ankle.
The foot is planted, as the weight comes forward, its the only point that will not move.

But i am also a hella good shot and hella good at human mechanics.

Also, i would never be standing like this guy.
If i was actually close, kick his right foot as he moves his knife shoulder forward and he falls to the ground.
Then neck stomp him until he no longer moves.
Don't let him roll over to get his knife hand out from beneath him