I have a few different ones, mostly I use a 5X Bausch & Lomb Aspheric however if I'm looking at an error or something that I need maximum eyes. I pull out the jewelers loupe and it's 16x power!! A bit much for sure but it's always better to be prepared.
I think we have had this type of thread before but it is always good to see others' opinions. I personally like 8x --it is small enough that I can see the whole coin at once but large enough to see marks, particularly hidden ones in the Indian's hair on a Buffalo or in the Eagle's feathers on a Walker, for instance. I do have a few 5x's laying around also. I have a 10x that I used to use but not much any more.