One of the best taunts is the rancho relaxo taunt. It's a taunt for the Engineer. He basically sits on a chair and drinks soda.
The second best taunt in TF2 is the Katzi Kick. It's a Russian dance. Every character in TF2 can use this taunt and dance.
The third best taunt is the Rock, Paper, Scissors taunt. Basically when someone comes up to you then you can play Rock Paper Scissors with them. Who ever loses blows up and dies.