Most overrated: Dragonball... I don't understand the hype... It's cool, but it's not as great as those outside of the anime scene get the impression that it is. It's about the only talked about anime outside of the serious anime people.
Most underrated: Initial D. It doesn't get much exposure in the states. Inuyasha as well, but not as much (as far as I can tell).
P.S. Add Pokemon to the underrated list. People consider it childish, but it's amazing. At least that's my opinion. A lot of people don't seem to share that opinion with me.