For me, it has to be Colby. The mix of sharp and creamy is awesome. Makes great Mac and cheese too.

And on rye it makes a great grilled cheese sandwich.
 
Colby, Swiss, Cheddar, Muenster, creamy blue…for me they’re all good on (and in) a cheeseburger.  American is often too bland for me, though.
 
What I can never figure out is when to capitalize the name of a cheese or leave it lower case!