Well, the "correct" answer would be whichever coin works best for the method for you.
Personally I do very little coin magic but when I did, I generally only used half dollars. The reason for that is simply visibility. I wanted everyone to be able to see the coin, otherwise there's no magic.
These days if I were to develop a coin based routine I'd probably use challenge coins, which are generally around the size of a silver dollar. Again - visibility.
However, you also have to consider things like whether you're going to use pure sleight of hand or gaffs, etc. If you use gaffs you're going to be limited by what kind of coins you can get those gaffs made in. You can, obviously, get gaffs custom made but that's going to be pricy.