Favorite developers? That's easy.
Nintendo. In general, they put out great games in the past. Of course, recent ideas have been going a bit off lately, but they still manage to release awesome games in this age, Like The Legend of Zelda: Skyward Sword and New Super Mario Bros. Wii.
Game Freak. Unsurprising, considering I'm a member of this forum. XD But seriously, if I had to pick a franchise to be my absolute favorite, Pokemon would be it. From childhood and ongoing, I love the series. If there never was a Pokemon game released, I'd be an entirely different person right now!
HAL Laboratory. They make the Smash Bros. games, afterall. The Kirby games are also great, but some recent wones have been released on the downlow, which upsets me. Other titles, like Kirby 64: The Crystal Shards and Kirby Super Star (and it's remake) have made me an extremely happy boy in my time!
Valve. Probably the only developer that has no ties to Nintendo (at least a big-time developer). My favorite series would have to be the Portal games, for being briliant with a very limited cast. Other games, such as Team Fortress 2 and Left 4 Dead, are fun games to play with friends or others online, and have a nice amount of comic relief and awesome/balanced-out gameplay, keeping things fair. I'm currently playing the half-Life series. I'm scared sh*tless of the headcrabs, but otherwise, I'm enjoying the story. (I want a headcrab hat, for some odd reason. Is that bad?)
A small-time guy, but still worthy of mention, is Jonathan Blow. Creator of the videogame braid. So far, that's his only videogame on the current market, but it's briliant, fun, metaphoric, poetic, and head-scratching all at the same time. Easily one of my favorite games of all time.
I would've mentioned Sega, but I feel that their only contributions are Sonic games, while most of their other works are living in shadows for the most part. Other developers I'd like to get my hands on would be Konami (specifically for the Metal gear Solid titles), and SCE Santa Monica Studio for the God of War games.