Well, I'd definitely want people with regenerating health or shields because I'd be hogging the medical supplies like a fat kid with the twinkies. Also, you have to have people who aren't going to turn on you, so no evil buggers. Also, people with melee or unlimited ammo for one or more weapons are a must, because I live in Canada and we don't have spare ammo and .50 caliber machine gun emplacements conveniently located on every block. With all that in mind, I'd go with the following:
1. Alex Mercer from Prototype - he could not only kill the zombies, but also eat them and regain health in between waves.
2. Mace Windu from any Star Wars game - Although I'd have to resist the urge to say "use the force, mutha fuckka!"
3. Kratos - If you can knife Zeus in the ribs, you're on my team.