The best and most widely used taunt is constantly pressing and releasing down so that your character keeps squatting over and over. It's great when you get a triple kill in FFA, and even better when two teammates do it after a double kill in 2v2.
I remember one time in a quickmatch 2v2, my teammate was eliminated and I had one stock left, and while the other guys were waiting for me to respawn, they were doing the squat taunt, and after I respawned, I joined it. The three of us did that for like 2 minutes before bringing the match to a close. It was great.