Without considering perks, any Hand Cannon is a 3-headshot kill. In this scenario, a low impact Hand Cannon like The Devil You Don't has the lowest TTK.
Luck in the Chamber and Final Round boost damage by 33% though, making it possible to get a 2 headshot kill with a medium-impact Hand Cannon like The Devil You Know or Red Hand IX.
Without lucky bullets:
Low Impact (TDYD, Revelator, Up The Ante): 0.73s
Middle Impact (TDYK, Jewel, Fatebringer et al): 0.86s
High Impact (Ill Will, Timur's Lash): 1s
For reference:
High Impact Pulse Rifles (Messenger, Pilgrim): 0.73s
Middle Impact Pulse Rifles (Allfate, Red Death, 9th Edict): 0.67s
High Impact Scout Rifles (Scholar, Ghaleon's): 0.8s
Middle Impact Scout Rifles (Fang, Salvation State, Rapier): 1s