Well there would be three types of people you would need. A front line fighter, a backup (sniper type), and a support. Here are my top five from each category.
FRONT LINE
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*** The Heavy from TF2 - Sheer crowd leveling ability
*** Soldier class Captain Shepherd from Mass Effect - Heavy armor makes him pretty much immune to attack and infection, and unlimited ammo in his shotguns and assault rifles
*** Tauren Warrior from World of Warcraft - Heavy armor, proficient with large blunt weapons, and thunder stomp ability useful for crowd control
*** Master Chief - Full armor grants immunity to infection and attack, strength and jumping ability makes him virtually impossible to bog down
*** Big Daddy/Project Delta from Bioshock - Fully armored, drill hand, and useful plasmid abilities
BACKUP
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*** The Sniper from TF2 - Well, um, ya
*** Infiltrator class Captain Shepherd from Mass Effect - Unlimited ammo sniper rifle and pistol, effective combat medic
*** Dwarf Hunter from World of Warcraft - Good racial abilities, and traps are useful
*** Captain Price from Modern Warfare - Great sniper and tactician
*** Sweetwater from Battlefield: Bad Company - Experienced in "sh*t hitting the fan" scenarios, comic relief
SUPPORT
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*** The Engineer from TF2 - Sentries useful for nappy time, dispensers for health and ammo
*** Biotic class Captain Shepherd from Mass Effect - Crowd control abilites, excellent medic
*** Undead Priest from World of Warcraft - Can pass for zombie, Shackle Undead, good mix of offensive/defensive/support abilities
*** Stubbs the Zombie - The ultimate zombie undercover agent, mind-control hand is useful against hostile humans
*** Sam & Max - If you have to ask, DIAF