Always butter, but like @Simon, I am intrigued by the mayo. I plan ahead and take the butter out of the fridge early in the day so it's spreadable by the time I make the sammiches.
I also like to pan sear pieces of ham and add thinly sliced tomato to my grilled cheese.
American and cheddar (on the same sandwich) are my go to cheeses.
Love me some nice tomato soup, but I have also been known to dip my grilled cheese sandwiches in ketchup. Don't be a hater and don't knock it till you try it.