I got a great recipe out of the Fannie Famer cookbook; I believe it calls for onions and breadcrumbs sprinkled on top. Heavenly. I think my mom used to use sharp cheddar; of course when I was a kid I would only eat the packaged crap.
Don't overcook the macaroni before you put it into the oven; it will absorb more liquid there.
Use as much flavorful cheese as you can stand; a blend of mild and sharp cheddar, parmesan and maybe a bit of mozzarella for the top...

and always eat a salad or crunchy veggies with this meal; pasta and cheese can be constipating!