1. Frank West.
I don't need anyone else. He's covered wars, y'know.
/dodges rotten tomato
Fine, fine.
1. Roland from Borderlands - Capable of filling in as a combat medic while also resupplying the team's ammunition. Additionally capable of doing reasonable amounts of damage on his own and deploying a temporary defensive wall and turret should things get rough.
2. Leon S. Kennedy from the Resident Evil series - Experienced in the fine art of zombie killing (both slow and fast variety!), and will keep the team's morale up with his cheesy one-liners.
3. Sam Stone from the Serious Sam series - Certified badass with a huge variety of weapons. Unlike Duke Nukem, Sam hasn't gotten soft over the years, and is likely a valuable asset in holding off any manner of invaders, regardless of size or number.