Here are some of my favorites, with ratings.

= Riotously funny, almost made me have a car accident.

= Got a good belly laugh, would have spit up my beverage if I had a mouthful at the time.

= Chuckled a few times, maybe repeated the line to a friend while drinking, when it seemed much funnier.

= Made me smile, but that's about it.
= Not funny, had me seriously thinking about running the owner off the road.
"Somewhere in Texas, a village is missing its idiot."
"Guns don't kill people,
I do." (on a pickup truck)
"I brake for no apparent reason"
"My imaginary friend thinks you've got problems." (in erratic writing)
"No God, No Peace. Know God, Know Peace."
"I'm trying to see things from your perspective, but I just can't get my head that far up my ass."
"Don't blame me, I voted for the other guy." (I swear the next car I see with that one is going off-roading)
"We're spending our children's inheritance."
"Work Harder: Millions in Welfare Depend on You"
"The Lottery: A tax on people who are bad at math."
"
CAT! The other white meat!"
"Keep honking, I'm reloading."
"My kid beat up your honor roll student."
"I'm not losing hair, I'm getting head."
"My other car is a Lamborghini."
"Dog is my copilot."
"If you're going to drink and drive, make sure you have a car."
"Mean people suck."
"Beat rush hour. Leave work at noon."
"Warning: In case of rapture, vehicle will suddenly become unmanned."
"Come the Rapture, can I have your car."
"CAUTION: I can go from 0 to bi+ch in 2.5 seconds!" (I actually knew that chick, and it is totally true)
Honorable Mention:
"Can't Feed 'Em? Don't Breed Em." (Not especially funny, but get's an "A-Men" from me)