I generally just go for the classic cheddar on white bread. I hate Wonderbread most of the time, but I really think its great for grilled cheese bread,. Of course, I use LOTS of butter to make it crispy and amazing.
I do like to go outside the box a lot though, too. Today I tried it with mozzarella and pineapple, and it was delicious! Mozzarella is also good combined with fresh basil and tomatoes on Italian bread (with a little fresh garlic rubbed on it). In the latter case I use olive oil instead of butter.
So how do you make it?