Depends on how many letters/numbers would be allowed on a plate. In real life, there's so many restrictions (some states here are only 6 letters, others allow 9) so it would be a toss up as to what I would write. But here's some suggestions for the 6 letter size, some are neat, some are a little risque:
U LOST (You Lost.)
UR2SLO (You're 2 slow, my mate has it on his S14 w/RB26.
)
CACHUP (Catch Up.)
CACHME (Catch Me.)
DBLNDA (Double Ender, yes I'm immature.
)
DAAAMN (Damn in Chris Tucker voice!
)
BUZZ4U (I'd fit this to an RX7!)
PUNTER (To lull people into thinking I AM a punter and make way for easy passes!
)
ILRAMU (I'll Ram You, For the same purpose, to fool people!
)
IOWNDU (I owned you.)
IDO07S (I do 7's, to illustrate my cars awesome pace!)
Anymore than this and I'll be doing a novel.