Nintendo, without question. They're the only one seemingly able to support two platforms at all well simultaneously too (looking at the semi-pathetic lineups of SCE on PSP and MGS on PC in recent years). Seriously, no other first party develops as much content as Nintendo, sells as many games as Nintendo or seems to have the industry wide influence and impact of Nintendo. Nintendo also works directly with more outside developers on 1st party content than Sony and Microsoft combined, and also hold the distinction of being the only 1st party who lets their own teams make games for 3rd parties directly. They are truly unequaled, and haven't really been (imo) seen Sega's heyday.
And for the FULL list of 1st party Nintendo studios (and the bigger recent games they worked on)...
- Nintendo EAD Dept 1 (Nintendogs, Mario Kart DS/Wii)
- Nintendo EAD Dept 2 (Wii Sports/Resort, Animal Crossing WW/CF, Wii Play, Wii Music)
- Nintendo EAD Dept 3 (Zelda series: Twilight Princess, Phantom Hourglass, Spirit Tracks)
- Nintendo EAD Dept 4 (Big Brain Academy/Wii Degree, New Super Mario Bros/Wii, NPC Pikmin 1-2)
- Nintendo EAD Dept 5 (Wii Fit/Plus)
- Nintendo EAD Tokyo (Super Mario Galaxy 1-2, Flipnote Studio, NPC DK Jungle Beat)
- Nintendo SPD Dept 1 (Rhythm Heaven, Tomodachi Collection, various Wario Wares, Metroid Other M)
- Nintendo SPD Dept 2 (Tetris DS, Style Savvy)
- Nintendo SDD (Brain-Age 1/2, Jam with the Band, English Training)
- Nintendo NSD (Personal Trainer Walking/Cooking)
- Nintendo Software Technology (Metroid Prime Hunters, Mario Vs DK 2/3)
- Intelligent Systems (Fire Emblem RD/SD, Super Paper Mario, Advance Wars DS/DoR, DQ Wars, various Wario Wares)
- HAL Laboratory (Kirby Canvas Curse/Squeak Squad/Superstar Ultra, Pokemon Ranger 1/2, 3D Picross)
- Creatures Inc. (Project Hacker, PT Walking, Pokemon Ranger 3, PokePark Wii)
- NdCube (Wii Party)
- Retro Studios (Metroid Prime 3/Trilogy)
- Brownie Brown (Mother 3, Layton 4's London Life, Heroes of Mana, Blue Dragon Plus, Kappa's Trail)
- Genius Sonority (Pokemon Trozei/Battle Revolution, Dragon Quest Swords, 100 Classic Books)
- Monolith Soft (Disaster, SRT OG Saga 1/2, Soma Bringer, Xenoblade)
- Project Sora (Super Smash Bros Brawl)











