Rare. DK Country put them on the map, GoldenEye made them immortal. But what after that? Perfect Dark was great ideas held back by technology, but since it has all been downhill. Conker is more just for the comedy, but as a game it is merely ok.
I think most overrated has to be Rockstar. I appreciate what they have done for the industry with the "sandbox" type game, but I think the GTA series is WAAAAAAAAAAAY overrated. If it didnt have the drugs, sex, language, and violence, probably only 10% of the people who play it now would play it. I am very curious to see what they can pull off outside of GTA or going for the ultra violence.
Now, in defense of some others.
Team Ninja. Dead or Alive and Ninja Gaiden were awesome. They havent branched out much from there, but there will be soon. I love them, but they arent worshipped enough to be overrated.
Starbreeze? Again, not enough done or enough unbiased love to be considered overrated.
I think to be considered overrated it should be a developer that sells games on their name alone and have almost universal praise and blind admiration. Developers that fall into that are Nintendo, Sega, Capcom, Rockstar, EA(i know they are a publisher, but how often do you hear them giving props to the developers?), Rare, Bungie, iD, Blizzard, Valve, Kojima(or whatever his studio is called).