The absolute best I’ve seen was posted by CalMeacham in theWatership Down thread.
INLE on a black VW Rabbit.
I guess I didn’t really see it. I guess I am guilty of thunder-stealing. But it is so good!
About a year ago, Mr. S and I were walking through a parking lot and came upon a somewhat decrepit Delorean. Cool! Never saw one up close and personal, so we briefly checked it out. Got around to the back and just about pissed our pants laughing when we saw the license plate. I even took a picture of it:
FLUXING
My favorite from at least 10 years ago in Lincoln Nebraska - a (from memory) orange Cobra with the plates It Bites.
Just saw this one this morning… SUM EGO. Don’t know what breed of car, but I guess it’s all about the driver, anyhow.
Years ago, on a convertible:
“OBN OHNE”
I thought nothing of it, but a passenger in my car started snickering. She was from Germany, and evidently “Oben Ohne” (I know I’m mangling the spelling, sorry!) means “Topless”. Ideal for a “topless” car 
I’ve always wanted to see if I could get 3M TA3, but have never gotten around to it.
So that was you I saw in my rear view mirror!
Oh, the Cobra one reminds me of the other day, when I saw a Cobra with the plate VENIMUS. It took me a minute to figure out that it was “venomous,” not Latin. Anyway the whole thing just screamed mid-life crisis.
I actually saw TIH2 O on a car with MD plates on the Beltway several years ago. I wonder if DMV ever figured it out.
Also, on a Honda Prelude: NFUGUE
Had a friend with a black '63 Continental (a huge car) with the plate EATS VWS.
The best one I’ve ever seen was on an airbrushed, surfboard-laden panel van (shaggin’ wagon, fuck truck). It simply said:
GET IN
On a white VW Rabbit:
IML8
It took me way too long to figure that one out.
I used to work for a large corporation so I don’t know who had this plate, but I saw it right around the time the first Lord of the Rings movie came out. It said ARAGORN.
I suspect the guy was a huge Tolkien fan who suddenly found himself tres cool.
Saw one a couple weeks ago that I really appreciated.
It was on a black Grand Marquis, and read:
DRK MRK
Double puns…gotta love 'em. 
My husband and I saw this on a minivan this weekend:
VAN GO
I generally hate vanity plates, but I thought it was cute.
Back in college I saw the plate SLYASA. I didn’t get it until I saw that the car was a VW Fox.
oh, i just remembered … my brother’s friend had SLAAB on his (you guessed it) SAAB.
My FIL has a '97 Purple Plymoth Prowler. We voted that his vanity plate should read “PRPLROD” for purple hotrod, but the DMV said no…
My wifes aunt’s name was Virgina, she wanted SLO GIN on her car, but again, the DMV said no… Bastards…
This one is always a classic:
My favorite so far is one I’ve seen around where I live, and that I’ve posted here a couple of times: DCY4THS.
My old tags used to be JE VAIS (French for “I go”), and one day I was passed, appropriately, by a car with the tags JE FILE (“I slip by”). The other driver noticed my tags, too, and as he passed we nodded at each other. 
I’m pretty happy with my current tags, which a few fellow Dopers (especially rowrrbazzle!) helped me figure out: 867-V309. 