Dohohoho. Oh, you.
I'll admit, I do hate the Spy's taunt for a number of reasons. The fact that it hits twice before connecting with an instant kill is one, giving away your position (which is something that you as a Spy should know better than to do).
The other is the fact that the Spy already HAS an instant kill that teaches an opponent to pay attention to their surroundings, and that's the backstab.
With that said, the Scout's Sandman taunt may not be the most useful--After all, a Scout can kill an unsuspecting opponent with a close-ranged FAN shot. But I'm pretty sure the Scout's taunt, of all of them, was meant to be the most satisfying to pull off. That's why it's so long, and why it's got the most satisfying result.
The Hadouken is quick enough to take care of unsuspecting Snipers and the like, so it's pretty good overall. The same with the Huntsman's stab, although an opponent can possibly slide out of the way after being stunned. Rare, but it's extremely quick and silent, which makes up for it.
The Heavy's Fists taunt has to be one of the most useful however. It's slower than the Huntsman stab, around the same length (maybe a little slower) in speed as the Hadouken, but it has range. The Heavy can take an initial hit and survive usually, unlike the Sniper, Spy, or Scout. Therefore, I believe it's the best taunt.
*Deep inhale and sigh*
And THAT'S my opinion of the taunt kills.
