About 30 years ago I was waiting to cross a junction in the centre of Lincoln, when a Rolls/Bentley (not sure which) wafted past wearing the plate '1 RAF'. Very impressive - always remembered that.
I have seen a Porsche 911 with 'V1 V2 R', and at Shawbury there was - maybe still is - a bootie pilot with 'FLY RM' on his car.
Best plate I ever saw was not aviation related. At a carshow there was an original Lamborghini Countach with a front plate which was 'T13VOM'. It took me a minute to get it - when you see it behind you in the rearview mirror, it reads 'MOVEIT'.