I'm not going to claim that Riva's (1117 Missouri, 529-3450) serves the best meatball sandwich ($4.50) in the whole damn solar system. That would smack of exaggeration. And besides, it's years since I've been to Mars -- though I hope to pop over when I visit Saturn next July. All hyperbole aside then, let me put it like this: Riva's makes the best meatball sandwich I've ever tasted. And that's saying something, because I've put away a good many in my time. For much of my life, I've lived on nothing else. Riva's serves what it calls an open sandwich. Bread and meatballs -- the latter with lots of mozzarella and a truly classic tomato sauce -- are served separately, which means you'll have to assemble it yourself. But don't worry. It doesn't require a screwdriver. And even better, it's utterly, blissfully delicious.