1. Ton Kiang especially for their dim sum. It's not downtown though -- it's in the outer Richmond area though. Very convenient if you're coming down from Sonoma though and want to avoid downtown.
2. House of Nanking for best chinese (and probably one of the cheapest) in SF. You probably want something fancier than this though.
3. Thanh Long for best Vietnamese seafood. Their roast crab and garlic noodles are the best I've ever eaten. Also very convenient if you're passing through SF from north to south.