Intelligent Systems—the guys behind Advance Wars, Fire Emblem, WarioWare and a bunch of entries in Nintendo's first-party franchises—have consistently put out really polished titles with more under the hood than you'd expect. Easily my favourite handheld developer over the past few years; I still dig out Dual Strike every few weeks in spite of the game being four years old. Plus, they're responsible for Super Metroid, too, so there's always that.
Quick mention of
Harmonix for carrying on the top quality of the first two Guitar Hero games by starting a new, superior franchise after Activision picked up the license.
Also, while they're not a "top" developer by any stretch of the imagination,
Treyarch actually showed a lot of promise with World At War. Despite essentially being a reskinned Modern Warfare, the game had a shitload more polish than CoD3 and the awful Bond game they put out. The amount of community support was good to see, too; they took suggestions for new content to heart, patched out a couple of egregious exploits in the Nazi Zombies game, and a couple of the developers still regularly interact with people on twitter. I'm not really sure I can quite say they've redeemed themselves (I owned Spider-Man and Minority Report at one point

), and they're nowhere near the same level of quality as Infinity Ward, but they've improved a good deal recently. Maybe they'll actually, you know, come out with an original IP someday.
edit:
nthing the mentions of Valve, Rockstar and Treasure, none of whom have ever released a game I didn't enjoy.
Jam it back in, in the dark.