I saw a Land Rover Range Rover with the license plates 'GOFAR' many years ago.
Anyways, if you want to infuriate your local environmental community, exaggerate the kind of numbers your car get. For instance, if you have a SVT Raptor, you'd want to get something like '6MPG' or '701GCO2' (701 grams of CO2 per mile).
Plates I was considering for my Subie (later on in the year, that is) are 'TRUSTI' or 'LASOUND' ('La' being 'The' in Spanish; The Sound of the Boxer engine).
Other plates would be:
AZNBENZ (Asians who own a Mercedes-Benz)
ONDALT (On the Left; referring to you passing another car on the right side)
WTHAEVO (What the Heck is an Evo (yes, grammatically incorrect, but clever, nonetheless))
GOBRAKE (Concept is that if they see your license plates, they should just brake because they're going to lose anyways)
DEMFEEL (How does it feel to lose)
4DRS or 5DRS(4 Door Sedan, 5-Door Hatchback; a statement you can make when a Camaro SS loses to you)
ROAHHHH (Sound of a Boxer engine)
4CYL (Another statement you can make when a Camaro SS loses to you)