SquareSoft and Enix, not to be confused with SquareEnix.

This is hard as hell for me since I'm such a big fan of so many genres and on so many different systems.  I could list 20 developers easily who I've had great times playing their games. 

However when all is said and done there is one developer who has always delivered quality and I've spent more time playing their games probably more than any other.  The great and fantastic

but honorable mentions would be

Square-Enix

Nintendo

Epic

Naughty Dog

Sucker Punch

Santa Monica

Microsoft 1st party

Ubisoft

and many, many, more

My favorite developer of the past -- Bungie

My favorite constant developer -- Bioware

My Favorite overall developer -- Bioware! Seriously any developer that can make gaming Nirvana like Mass Effect deserves so much praise!

Honorable mentions are:

Codemasters

Santa Monica

Ubisoft

Last gen it would be Bungie with Halo(obviously). This gen it would have to be Valve. There has not been a single Valve game that has disappointed me.  The Orange Box, Portal 2, L4D, and L4D2 have all been excellent games. Overall Developer would be Bungie.   

Honorable Mentions would be Bioware(Mass Effect series), Epic(Gears of War series), Ubisoft(Assassin's Creed series), Turn 10(Forza series), Remedy(Alan Wake), and Rockstar(GTA4 & LA Noire).          

Nintendo is by far my favorite, followed by the old Rare. =)

I will give you my top 5 

1. Insomniac 

2. Bungie 

3. Rockstar 

4. Naughty Dog 

5. Square Enix

Honerable Mentions 

1. Ubisoft 

2. Epic games 

3. Bethesda 

4. Infinity Ward 

5. BioWare (Jade Empire, Mass Effect, Dragon Age) You know what BioWare deserves to be in my top five games lol. 

I'm still a Nintendo guy at heart, so they get my vote. I will say that I've been getting pretty attached to Mistwalker recently as well.

Rockstar Games!

Not my current list of favs but my favs if we count all the games in their libraries from a given time (this lis is BIG publishers):

1. Square-Soft (FF6, FF8, FF7, FF9, FF10, FF4, KH1, Chrono Trigger, Xenogears, Secret Of Mana)

2. Konami (Metal Gear 1,2, MGS1,2,3,4, Peace Walker, ZOE, Castlevania)

3. Nintendo (Super Mario World, DKC1,2, DKCR, MArio Galaxy, Maro 64, Zelda OOT, Zelda Wind Waker, Zelda Spirit Tracks, Smash Bros Melee, Brain Training, Metroid....)

4. Square-Enix (FF13, FF12, KH2, KH:BBS, TWEWY, Dissidia, FF7:CC, Dissidia, DQIX, Chaos Rings, Just Cause 2)

5. Capcom (RE1,2,3,4, Ace Attorney Series, Ghost Trick, Breath Of Fire, DMC)

Honorobale Mentions (SCE, Ubisoft)

________

Top 4 independant devs devs:

1. Funcom (The Longest Journey, Dreamfall : The Longest Journey)

2. Rockstar (GTA3, San Andreas, Bully, LA NOIRE, RDR)

3. Atlus (PErsona 3,4, Shim Megami Tensei Devil Survivor)

4. Bioware (their independant days) ... KOTOR 1,2, ME1, Jade Empire