I’ve always been fond of:

The person who makes me, doesn’t want me

The person who buys me, doesn’t need me

The person who uses me, doesn’t even know it

What am I?

Maybe not the hardest, but it’s a good one, and most people I ask, don’t get it very quickly.

I’m not sure how to spoilers the answer, so I’ll just give you time to guess the answer. It’s more fun that way.