It has got to be steak, baby! A choice cut of beef is my favorite thing off the grill, but really any meat is great when grilled properly. I often grill vegetables also, like potatoes, squash, corn, and so many others. I am a grill fanatic and grilling takes place at least once a week at my house.
I do a grilled smoked tofu, which I then use either cubed in stir-frys, or sliced into sandwiches loaded with "all the trimmings."
I also do a summer vegetable basket, with all kinds of squash, mushrooms, onions, lemon wedges and cherry tomatoes, all marinated overnight in Italian dressing. There are never any leftovers! It's a big hit at our annual summer "Burn Your Own Burger Bash." ;-)