Simple answer Square (you know before Enix "bought" them and made them SquareEnix.)
Because without them Sony would not be big at all. And Nintendo would have had more gamers. And without Sony Sega would still be in the console business and who knows if MS would have joined the fight at all. MS basically joined to destroy Sony.

So it all goes back to Square moving from Nintendo to Sony systems.

square enix? thats a bold thing to say considering how people feel about them on here  

I read the OP and saw of all time... Are you silly? There is no one 3rd party developer or publisher that can hold that title. Ninteno,SEGA,CAPCOM, Team ICO, SqaureSoft/Enix,Namco, Konami, Treasure.. list goes on. Each these developers has impacted market in long term in some way or another.

Valve ?

A C T I V I S I O N

Learn 2 gaming history, dudes :P

What engine does Call of Duty 4 run on?

LJN

Based on how many of their games i have played over the last 30 years....... i have to go with Capcom.

IW (Infinity Ward) 3.0 engine. It is basically a modified version ID Tech 3 engine by ID Software. First game based on ID Tech 3 was Quake 3 arena in 1999. 

I know, I was asking mr gaming history there :P

Edit: this might be of interest to you FPS fans out there -