My daily driver is my company car, so I don't tend to put any stickers on it. Just a "Silverado, California," sticker that is supposed to let me into the town during its latest natural disaster without my having to stop to talk to the sheriff.
But I do have a Disneyland "45 Years of Magic," license plate frame.
The worst bumper sticker I ever saw? I was driving in Orange County, CA, which is one of the more conservative parts of southern California. It was 1993, in the midst of the controversy about gays and lesbians serving in the military. A truck ahead of me had a bumper sticker which said,
"Roosevelt: A chicken in every pot.
Clinton: A fag in every pup tent."
And it was actually a company truck.

I hope they lost business because of that hateful bumper sticker.