I like: My other car is a piece of s**t too!

It just takes the piss out of the pretentious My other car is a Rolls Royce, or my other car is a sailboat.

This is a strange one to have on a vehicle emblematic of the Third Reich:

I'd rather be smashing imperialism.

And perhaps the densest one I've seen:

"Sorry I was late for church. I was learning witchcraft from my lesbian comrades."