I think the best one I saw was on a Chevy Train Blazer and it said - "Save gas, drive a motorcycle" I was like, yeah, and right now your guzzling all the gas with your Trail Blazer!!!!
Another one I've seen often is "Don't get to close, or I'll flick a boogie at your car"
The ones I HATE are the boy peeing on whatever. Especially the ones that are in color, and you can see the color of the... you know what. Whats the point of that?!??!