Strictly in terms of canon material I would have to go with the Pulse Rifle simply because it would be the one thing to keep me alive longest against a xeno.
However, in terms of fun, or if we are talking about weaponry in both the movies AND the world today, I would go with the Flamethrower. As you can see in the pic below, limited range isn't an issue in RL, and let's be honest: between the three weapons it's most fun to watch something burn...