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!

1. BioWare
2. Atlus
3. Square Enix
4. Naughty Dog
5. Capcom
6. Obsidian
7. Insomniac
8. Ubisoft
9. Rare
10. EA

God, a list of 10 seems a little excessive.

1. Blizzard
2. Rare
3. Nintendo
4. Criterion
5. Kojima
6. Infinity Ward
7. Obsidian
8. BioWare
9. Rockstar North
10. Bungie

10?

I'll just post 6.

-Square enix
-Bioware
-Bethesda
-Rockstar
-Capcom
-Atlus

Kinda hard since devs are always improving/changing, but here goes:
1. Nintendo
2. Bethesda
3. Firaxis
4. EA
5. Capcom
6. Ubisoft
7. Rockstar
8. Valve
9. Bungie
10. Rare
(Most are randomly placed, though, Nintendo is probably gonna stay number 1)

1. Harmonix
2. Bethesda
3. Ubisoft
4. Monolith
5. Blizzard
6. Valve
7. Bungie
8. Square Enix
9. Nintendo
10. EA or 2K games

1. Rare
2. Insomniac
3. Nintendo
4. Naughty Dog
5. Sucker Punch
6. Valve
7. Rockstar
   
8. Capcom
9. Square Enix
10. Atlus
    

Valve is pretty much the only developer that I completely trust. If it has their name on it, it's a day one purchase for me. IW is probably the closest second. Beyond that, its about the game, not the developer for me.

 1.Rockstar
 2.Creative Assembly
 3.Bethesda
 4.Blizzard
 5.Valve
 6.Ensemble Studio
 7.Bioware
 8.Ubisoft
 9.Capcom
10. EA

I dont have a favorite let alone 10

Why are so many of you naming publishers.

I know EA develop alot of games but EA is a publisher with an army of devs, and shouldnt be considered one.

Also Rockstar,       which one?,   OK i presume you guys mean Rockstar North (GTA), but again Rockstar is an umbrella of many devs.

Mine are probably,
in no particular order.

Rare  {Banjo, Viva Pinata, Kameo}
Lionhead   {Black & White, The Movies, Fable, Theme Park}
Media Molecule   {LittleBigPlanet}
Dice   {Battlefield}
Relic   {Homeworld, Company of Heroes}
Infinty Ward   {Call of Duty 4}
Ensamble Studio   {Age of Empires}
Valve   {Team Fotress, Left 4 Dead, Half Life}
Fraxis   {Civilization IV, Railroads}

In no real order and I only have 9 favorites. 

No particular order, only 7 of them.

Naughty Dog
Insomniac Games
Rockstar
Blizzard Entertainment
Valve
Infinity Ward
Sucker Punch

1. Bioware - for KOTOR, Mass Effect, Baulder's Gate, Neverwinter Nights, Jade Empire, etc...
   
2. Bethesda -  Elder Scrolls series and Fallout 3
   
3. Blizzard - Warcraft, Starcraft, Diablo
   
4. Level 5 - Dark Cloud, Dragon Quest, Rogue Galaxy, Professor Layton, White Knight Chronicles
   
5. Square Enix - Final Fantasy, Dragon Quest, Xenogears, Kingdom Hearts, Star Ocean
   
6. Atlus - Shin Megami Tensei series, Trauma Center,
   
7. Relic Entertainment - Homeworld, Warhammer 40K, Company of Heroes
   
8. Valve - Half-Life, CounterStrike, Left for Dead, Portal, TF2
   
9. Insomniac - Ratchet & Clank, Resistance: Fall of Man
   
10. Naughty Dog - Uncharted, Jak & Daxter

umm =\ some of you posted game publishers not developers, like EA and Ubisoft, no? :S

1. id Software - Wolfenstein 3D, DooM, Quake. The impact is immeasurable.
2. Volition - FreeSpace and FreeSpace 2 are more then enough to get to the number 2 spot.
3. Red Storm Entertainment - The original guys behind Tom Clancy's Rainbow Six.
4. Relic - Homeworld, Dawn of War, Company of Heroes... an impeccable tradition of RTS excellence.
5. Ubisoft Montreal - The guys who make the good Splinter Cells.
6. Valve - It's pretty obvious why they're noteworthy.
7. BioWare - Knights of the Old Republic is the best RPG I've ever played. Mass Effect may be a close second.
8. Raven Software - Reliably great innovators of id's engines.
9. Bethesda - They're known for the Elder Scrolls series, but I still remember when they made the great Terminator games: Future Shock and SkyNET.
10. Crytek - Crysis and Far Cry are much more impressive then some people give em credit for.

So hard to come up with a 'fair' list on this. It's only the small to medium developers that really stand out, because once it gets to a certain size it loses the hallmarks that make it distinctive. It's almost a measure of the developers that made few enough games to manage a good run in terms of quality.

Im a bit confused about people picking Bungie they have only made 1 franchise (to my knowledge) and while some people may regard the franchise as the best in the world ( i don't) i don't really believe they qualify to be called one of the worlds best dev's. Also some people need to know the difference between developers and publishers

"

Im a bit confused about people picking Bungie they have only made 1 franchise (to my knowledge) and while some people may regard the franchise as the best in the world ( i don't) i don't really believe they qualify to be called one of the worlds best dev's. Also some people need to know the difference between developers and publishers

"

"@eclipsesis said:

"

Im a bit confused about people picking Bungie they have only made 1 franchise (to my knowledge) and while some people may regard the franchise as the best in the world ( i don't) i don't really believe they qualify to be called one of the worlds best dev's. Also some people need to know the difference between developers and publishers

"

They've made more then one franchise. There was the Marathon series, then Myth, then... some game about a guy in green armor with a giant visor."

I can only think of 3 that I really trust and love their attitude etc.: Rockstar; BioWare and Valve. That's not to say I don't think there are other good developers, or games by other developers I love, but these are the only three I can think of that have given me consistently good and varied games i.e. I like Bungie, but only really for the Halo franchise and so it isn't in my head that a Bungie game will be great an interesting, just  "oh, Halo was great I hope this is good"

10? 10 is for overachievers. Here are 3:

1) IW - CoD 1/2/4
2) Square Enix - FF, TWEWY (never played a DQ game D: )
3) Bethesda Softworks - ES //would be 1st, but about 40% of the awesome that comes from playing their games (on the PC) arise from the fruits of labour of the mod community

I'll go with six instead.

1) Valve - I have yet to play a Valve game I don't like
2) Bethesda - Minus Oblivion and Fallout 3, some of the best RPGs ever
3) Troika - Revolutionary RPG devs.
4) Black Isle - Same
5) Rockstar - Great games
6) Bohemia - They refuse to dumb down their games for the masses

EDIT: Forgot Gearbox.

6) Bohemia - They refuse to dumb down their games for the masses "

Unfortunately true. But they do constantly release patches fixing most bugs.

1. BioWare - KOTOR, Neverwinter Nights, Baldur's Gate, Jade Empire, Mass Effect
2. Valve - Half-Life , Left 4 Dead, Team Fortress 2, Portal
3. Bungie - Halo
4. Bethesda - Elder Scrolls, Fallout 3
5. Ubisoft Montreal - Splinter Cell: Chaos Theory
6. Namco Bandai - Tales of Vesperia, Soul Calibur
7. Atlus - Persona 4
8. Obsidian - KOTOR 2
9. Oddworld Inhabitants - Munch's Oddyssey, Stranger's Wrath
10. DICE - Battlefield, Mirror's Edge

Top 10 Favorite Developers

1. Treasure - Gunstar Heroes
2. Capcom - They spam the crap out their IP's but they're still good!
3. Atlus - Shin Megami Tensei Series!!!
4. NIS: Nippon Ichi Software - Brave enough to import Really Japanese Games
5. Sega - Not current sega... but sega when they still had the Dreamcast!
6. Nintendo - It's like eating cherry pie all day, but who really gets tired of cherry pie?
7. Bioware - Neverwinter Nights... man that game was good.
8. Blizzard - They were a huge part of my life in highschool. Because of them I learned how to love... then turn marines into mini-nukes.
9. Konami - Not for metal gear... (sigh) But for Silent Hill and Azure Dreams!
10. Westwood Studios - The Legend of Kyrandia

1. BioWare
2. Rare
3. Bethesda
4. Bungie
5. Infinity Ward
6. Rockstar
7. Valve
8. The Creative Assembly
9. Epic Games
10. Ubisoft

ea sports
epic
valve
(whoever makes total war games i forget)
blizzard

• Capcom (namely for the SF series, but tons of other qualities franchises)
  
• Nintendo (Mario,Metroid, Zelda, among others)
  
• Criterion (Love the Burnout series and they are good to their fans)
  
• Bungie (Love the Halo series and they are also good to their fans)
  
• Firaxis (Huge Civilization fan)
  
• Atari Games (San Francisco Rush= the definitive arcade racing game)
  
• Rare (Consistently quality games, even if some aren't my cup of tea)
  
• Treyarch (Made Call of Duty 2: Big Red One which is my favorite CoD, also not given nearly enough credit)
  
• Ubisoft Montreal (Love Far Cry, Assassins Creed, the newest PoP)
  
• Epic Games (Fond memories of UT04, GeoW is quality)
  

Bungie

1.Capcom (I love pretty much anything they make) (My favorite character roster,best fighting games in the industry)

Double Fine Productions
Nintendo
Blizzard
Bioware
Valve
Rockstar
Square/Enix

Can't rank them... O_o

No Remedy or Starbreeze in this thread? That's sad.
Some of my favourites:

Black Isle
Interplay
Troika
Remedy
Starbreeze
Bioware
Crytek
Epic

1. Square Enix
2. Blizzard
3. Relic
4. Capcom
5. Valve
6. Nintendo EAD
7. Intelligent Systems
8. Atlus
9. Obsidian
10. SNK Playmore

Valve
Troika
Bioware
Blizzard
Team ICO
Relic
Looking Glass
CD Projekt
People Can Fly
Konami

That's.....all I can think of.

" God, a list of 10 seems a little excessive. "

In no real order: