Human NPCs:
Front - 2
Front Below - 1
Left - 1
Right - 1
Back - 1
Above Back - 1
Above Front - 1

Normal Headcrab:
Any side - 1

Normal Headcrab Zombie:
Front - 2

There's like a 70% chance that you'll get the 2 well-animated KillMoves, but 30% you might do the normal Human Front Killmove. But I might be wrong.

Fun fact, I actually discovered these 2 KillMoves while I was just playing through Ravenholm with a shotgun...

Hunter:
Front - 1

Strider:
On the leg - 1

Total KillMoves: 13. The unlucky number.

Tell me anything I missed and I'll add them. Killmoves created by other people besides the official creators will not be counted.