Overrated: Gas Powered Games and Lionhead Studios: both companies weren't able to provide a single game that is not boring by its stereotypes, shallowness and a very minimalistic game options (Fable trilogy for example is a very linear and shallow thing, with all this marriage, property stuff being only a cosmetic thing that doesn't affect the game play at all) offering no diversity or any advanced options to choose from when it comes to their "rpg elements".
And Molyneux has a big mouth full of promises of how something will be awesome and original, yet he always delivers the same underwhelming experience.
It's hard to choose an underrated company for me. Most of my favourites are non-existant. One of the things I don't like about indie retro is, that they do not realize that back then, when the "retro" they follow was born, it was something that broadened and deepened the genres, adding new possibilities and new ways of a video game experience. Yet most of these indie games are just parasiting on the concept that was fresh 15-20 years ago, but are actually not pushing the genre anywhere now. + Writing isn't as strong as it was back then.
And one more thing about Blizzard. I am not a fanboy, but you may dislike their games, I can understand that (even I don't like all their stuff), but they are always able to publish polished material, not something half-baked (even though I don't know about D3, I've never tried it) in both concept and design and this is a rare commodity in this industry full of bugs, glitches, poor ideas and half-made concepts. And even if it doesn't look like it, since they aim for the mainstream audience, they still put a little bit of innovation in their games which is not always a good thing, but it's there.