Well it depends on what you mean by industry "leaders" there are alot of companies that arent as big but push the bars of gaming.
There are alot of branches known as 3rd party developers
Here are some of the big ones in my opinion not just in monetary value but in new technology and gaming
Epic
Created the Unreal Engines
Bungie
EA
DICE
Crytek
It's not about which companies are the biggest, its about finding the compnay you would like to work for the most, and going from there.
Tell me if that helped any?
Check out
www.gamedevmap.com
They have lists of developers, production companies and so forth in your area