French Onion Soup cheese

So, last weekend I made a big pot of my mom's fabulous French Onion Soup. I ended up experimenting with cheeses to go on top. I ran out of Provolone (which is my choice) and used a Mozzerella/Parmesean mix for my last bowl. I decided I prefer the Provolone but wondered what other people prefer on their French Onion Soup. Is there a cheese choice I have not mentioned? Do tell...