Flood Campaign
ODST team searching old ruins for something or other some shadow ops mission of sorts. ODST encounter Flood, members being taken out one by one. You continue the game as the lone ODST member, the mission is f**ked, you're in deep shit, you're only mission is to survive and get the hell out of dodge.
Despite all my ranting, this is what I would play the shit out of. Basically Area 51 for the PS2 and Xbox, but scarier and more Halo.