There are 4 1 hit KO Moves.Sheer cold,Fissure,Gullotine,Horn Drill.These all can make a Pokémon faint in 1 hit.What do you think about which of these is the best I think its Horn Drill

In my opinion its Sheer Cold.
The reason is that it can affect all types of Pokemon.
Fissure,Horn Drill and Gullotine can be unaffacted on the specific type of Pokemon but Sheer cold affects all.
The most intresting fact is that if lock on is used before the 1 Hit KO move,It will always hit the opponent.

Awesome technique.

I read it on Bulbapedia.