Cheese.
And I don’t mean that homogenized liquid-at-room-temperature Velveeta crap. I mean CHEESE. Cheddar cheese. Monterey jack cheese. Monterey jack cheese with those little flecks of jalapeño in it. Actual cheese.
Oh – and you can’t bake them. You have to heat them from above, so that the chips won’t burn before the cheese bubbles. Put 'em in the broiler, or in a toaster oven on the “broil” setting. (And watch them like a hawk when you do – the boundary between “done” and “burned” is about 15 seconds!)