The superior single topping is pepperoni. The best ones are the crunchy ones around the edges, or if you get double pepperoni the entire top layer should come out crispy. Olives also get an honorable mention for their versatility in pairing with most other toppings (though an olive-only 'za would be boring).
If you're talking about ingredient combinations, you've got to factor in the sauce as well. A personal favorite is the barbecue cheeseburger pizza, with barbecue for the sauce, hamburger, bacon, and cheddar. You could also add red onions if you're deranged. The margherita is also an understated classic.