This one made me laugh:
Jesus Loves You
Everyone else thinks you’re an asshole
I think the comedian Gallagher came up with that one.
There also were ads in the back of the National Lampoon for opaque ‘club’, ‘screw’ and ‘spade’ stickers of various sizes.
They were designed to be placed over other peoples’ bumper stickers, resulting in stickers that read, “I (club) my wife”, “I (screw) my dog”, I (spade) my dog", etc.
I like
Meddle not in the affairs of dragons, for you are crunchy and taste good with ketchup.
I don’t know why, but it makes me giggle.
Don’t make me send out the flying monkeys.
I saw a white station wagon once that had, painted across the back in black lettering:
If you ride my tail [license plate]** I will slow down.**
I used to have “Urban Assault Vehicle” on my 1974 orange-and-white Mercury Comet. Well, it was an assault on good taste, anyway.
Also had “I’m mad at you since you ate my dog!” Surreal humor, considering I didn’t even have a dog.
When I lived in Santa Cruz I was always partial to the “Jerry’s dead, Phish sucks, Get a Job” sticker. But my alltime favorite was on the back of a dilapidated old VW van:
“I beat up three hippies and all I got was this crummy bus.”
"Sorry, my karma ran over your dogma"
I’m an avid reader of bumper stickers. 
The morning commute is so boring I have to do something.
Here are a few I recall seeing:
“I Brake For No Apparent Reason”
I’ve also seen the “Visualize Whirled Peas” one.
“I’m not losing hair, I’m getting head.”
“Clean up politics - elect women!”
“Beer: It’s not just for breakfast anymore.”
“Dyslexia is fnu”
“Going crazy - wanna come?”
I laughed out loud at this one. It was on a huge Suburban.
“Daddy farted, and we can’t get out!”
I know, I know. :rolleyes:
“Somewhere in Texas there’s a village missing an idiot.”
I love that one.
Not a bumper sticker, but a license plate holder:
If you touch my Saturn
I’ll kick Uranus
My favorite:
“Jesus was my co-pilot, but then I crashed into the mountains and was forced to eat him.”
Also, the “I <3 Manatees. They taste like chicken.” and other variations are pretty popular in the Florida area.