I was always a bit partial to:
"I got a gun for my wife. Best trade I ever made."
"My kid was Inmate of the Month at the Oklahoma County jail"
"Don't like my driving? Call 1-800-EAT-****"
"Visualize Whirled Peas"
and finally, the king of all bumper stickers:
"Your kid may be an honor student, but you're still a dumbass!"
Chris in VA, I actually had a local company make up some long banner stickers that said "What Would John Moses Browning Do?" - Had one on my pickup for awhile, but not many people really got it, so I took it off.