Generally I go with "asshole" and "Use your fucking brain!". Let me tell you, 90% of the time I get pissed at someone doing something asshole-ish in traffic, it's some tough guy in his pickup truck. What is it with assholes and pickup trucks??? The other 10% it's some jerkoff rich prick in a BMW.
The one traffic thing that I get into people with is when it's the drive home, and there's a lane reduction up ahead. Most people merge ahead of time, but there's always the fucking asshats who have to speed all the way to the end of the merging lane and then try to squeeze in. I can't tell you how many exchanges I've had with those fuckers.