Akira is one of the few I thought was overrated. Even when I first saw it in the 90's, I was like ok this is cool, but why is everyone going nuts about it, bigger question is, why do people still go nuts over it?
Naruto (the anime) is also overrated. The hype was deserved when the show first came out, and for the first 60-70 or so eps. But now it's blah and fillers.
Underrated, well a few months ago I would have said Nodame Cantabile. But it seems like it's getting well deserved attention lately.
Now this is going to sound crazy, but Lucky Star is somewhat underrated to me. The people who see it, love it. But the problem is, most people can't get through the first ep of basically nothing + a little character development and thus bash it. I never bashed it myself, but I had to force myself to watch the first 3 eps and as I watched more and more eps I got used to the style, dialogue and non stop talking which seems to be the turn-off. And rightly so, if it wasn't for fanboys praising it, I probably would have stopped after the first ep. It has now become one of my favorites.
Ichigo Mashimaro seems a bit underrated to me too. I could be biased, because I think the show is hilarious, but I never see many recommendations or chatter about it.