Valve, Rockstar North and Bungie.
Valve - what can you say? All of their games practically turn to gold in their eyes. They provide constant major updates, have given us some amazing games (HL2 and Eps, TF2, DOD:S/CSS) and generally care about their fanbase.
Rockstar North - again, what can you say? They've made some of the most amazing games of the last 10 years and got even better with GTA IV. They listen and care about their fanbase and improve on their upcoming games based on what the fans say.
Bungie - Albeit you have to pay for them (although that's more MS's fault), the map packs and constant support is great. They made 3 great Halo games and made some other great games in the past. They're adding on a campaign expansion next year to a game that's almost a year old now. That's dedication.
Honorable mentions:
Criterion - Their amazing support for a game that came out a while ago (Burnout Paradise) is great. Really shows they're not the typical money-grubbing dev.
Harmonix - Rock Band and Rock Band 2 have proved to be a LOT of fun for me, and they started off the entire GH franchise.
Epic Games - Gears was incredible, the UT games are great, and Gears 2 looks like it's going to be one huge gorefest.
I'd say Bethesda, but it wouldn't be fair for me to rate them on how awesome Fallout 3 is going to be..