The one that are most stupid are those where the only person who could possibly understand their significance is the owner. What's the point. Please: If you have to explain to other people why your registration number is so brilliant .... it isn't.
I think people with personalised number plates should have the number tatooed to their foreheads.
Having said that ....
I have seen a couple that were good. There was an estate agent in Cardiff that used to drive round in a rolls with :HOU5E, and I once was 1066AD.