This list does not pertain only to games made by these companies recently, but throughout their history.
It's also strictly based off of my opinion and personal biases (and is limited to and based off of games I have played), however, I'd love to hear and discuss why you agree or disagree or any comments you wish to leave. :)

 1. Nintendo- Zelda, Mario, Metroid Series (Ik Metroid has alot of outside involvement, and now with Team Ninja, but I had to include it)
 2. Blizzard- Starcraft/Diablo Series (I won't mention WoW, oh woops!)
 3. Rare- Goldeneye, PD, N64 Banjo titles, Starfox 64 (essentially all N64 Rare titles)
 4. Capcom-  All things Resident Evil :)
 5. Bethesda- Elder Scrolls, and Fallout 3 I suppose...
 6. Ubisoft- Splinter Cell and PoP Series
 7. Epic- Unreal and Gears of War Series
 8. Bungie- Halo Series
 9. Konami- METAL GEAR!
10. Bioware- Kotor/Mass Effect Series

*EDIT 7/6/09
*Thanks for sharing everyone! I know 10 may appear excessive, but I suppose it depends on how choosy you are (hehehe)
Updated my list by posting some highlight games for each developer, as many of you did...
And Firaxis certainly makes an honorable mention up there, I can't neglect the awesomeness of Civ!