Yeah, sniper is preferable because even if you taunt way after the other, you stun them and stop their taunt, also rendering them immobile until you kill them, plus it is very fast. Medic is good because it has the same stun effect while, but it takes a bit longer. Pyro is very good because pyro gives increased mobility with a taunt that is pretty fast, and can kill multiple enemies at once, along with buildings and stuff. Heavy is my taunt of choice due to his long range taunt, but it has a very small area of effect, and a relatively long cooldown (time he is immobile after the phase of the taunt that kills), downside being the fact that heavy is pretty slow moving, and a large target.
I would never use scout for a tauntkill battle because of the massive amount of time between starting the taunt and the actual kill coming in, although there is the massive advantage that every other taunt or so the scout can make himself invincible through the duration by drinking his bonk then taunting, plus his is freaking awesome ^.^
I wouldn't really use demoman because of the long cooldown time (longest I believe, although am not sure), and it still has a fairly long warmup to the actual kill, plus demo is slower and less maneuverable, although as you get heads by taunting you can actually become one of the most maneuverable on the field.
I would never use spy because of the massive amount of time it takes to taunt for the kill with him, and using the cloak taunt bug is forbidden while wearing tags
Lets see, spy, demo, heavy, medic, sniper, pyro, scout, who am I missing?
Oh yes, soldier, that one speaks for itself, if you don't have a medic healing you then you kill yourself, and it isn't worth it
