On a SUV - put on written upsidedown
'If you can read this, roll me back over"
Not necessarily my favorite, but it gave me a chuckle.
Have you seen this one?
W
The President
Well I saw this the other day, in the same style:
F
The President
I BRAKE FOR HALLUCINATIONS
667 - THE NEIGHBOR OF THE BEAST
And the little black tasteful one:
F
The President
My current favorite, and one that will be going on my car soon:
Republicans for Voldemort
Some I haven’t seen but wish I had (maybe I’ll get them made up for my car if I ever get around to it):
**My Goddess can beat up your God
Mean people suck
Nice people swallow
Creative people gargle
Practice random weirdness and senseless acts of originality
Death to all fanatics!
If guns are outlawed, how will we shoot the capitalists?
Librarians do it until the user loses interest
You are driving my other car**
It’s been a while since the days of the Big, Bad Bruins, but I still remember: Jesus Saves, but Esposito Scores on the Rebound
If you can read this, I can slam on my brakes and sue you!
So you’re a feminist… Isn’t that cute!
Honk if you love peace and quiet.
From back in the day, usually on a “hippy” vehicle;
“Don’t laugh - your daughter might be in here”.
And usually on an old couple’s motorhome, and one especially dear to me;
“Spending our kids’ inheritance” 
mangeorge
I have one that says:
I want to die peacefully in my sleep like my Grandpa
Not screaming in fear like the people in his car
(Maybe only funny if you knew my Grandpa.)
One I saw back in the '80s: My other car is up my nose
I have on on my truck that says it all:
I PWN NOOBS
It makes me laugh everytime I even think about it.
"If this van is rockin’, don’t bother knockin’.
I don’t know that they’re the best bumper stickers I’ve ever read, but all three of these made me chuckle today:
A small village in Texas is missing their idiot
Someone at Social Services needs a spanking
Don’t fuck with The Jesus
Stop Honking, I’m On The Phone
Ask Me About My Vow Of Silence
Caution! I Drive Like You Do
I haven’t seen this in 25 years, but it was a bumper sticker that just read:
BUMPER STICKER
Back when I was in college, it seemed that every other car had a “No Nukes”, “Save the Whales”, or “Jesus Saves” bumper sticker.
And then there was the one with “Nuke the Unborn Gay Whales for Jesus”
I just remembered my favorite one ever.
On the back of a school bus–STUDENT DRIVER
Spotted at a Hot Topic and sold out when I went back - dammit! -
"My kid beat up your honour student! "
“My high school dropout is sleeping with your honors student”
Hooo, that is so 1976! And, I remember that one. 
DON’T VOTE. IT ONLY ENCOURAGES THEM.
VOTE <insert party of choice>, IT’S EASIER THAN THINKING.