Taylor's helmet from Platoon has:
When I die, bury me upside down, so the whole world can KISS MY ASS!
And there are some other small ones like a grenaider from FMJ has:
I'm Hard
Or drawings are also cool, like Joker's bulls eye. Or and ace of spades like on platoon.
Half the fun tho, is making up your own
-Rafterman-