I would say Half Life, Counter Strike have to be at or near the top.

I'm also thinking Doom has to be up there as it was one of the pioneers of 3D, FPS gaming.

I'm not entirely sure about the Final Fantasy series but it must be one of the largest as well because aren't they up to FFX1VVX1Z or something similar?

Also, Operation Flashpoint also offered complete freedom over maps several hundred kilometers in size. They have sold millions of units and the game developer (BIStudios) creates real world simulation software for military applications under the title VBS.

Lets not forget other highly successful franchises such as:

- Tomb Raider
- Quake Series
- Battlefield Series
- Colin McRae Rally

I have a hard time believing that GTA has outsold any of the above.

Yes, right, I think Doom should be there too!!!!!!!!!!!!!!! It is really great game.

never played doom, as FPS isn't really my thing. what about microsoft's "age of empires" seris? it has to be one of the best "god games" or "strategy games".

I play AoE III, as does my daughter but I'm not sure if you'd consider it one of the biggest games of this genre. If I'm not mistaken, AoE III would be considered an RTS and I'd guess that it falls well short of other popular RTS's such as World of Warcraft, Command and Conquer, etc.

Speaking of WoW, here's a funny video if you haven't already seen it. It's worth the next few minutes of your time if your a gamer and enjoy a chuckle:

http://www.leeroyjenkins.net/leeroy-jenkins-videos.htm