Final Level: Destroy the Destroyer
Why It Sucked: So let me get this straight: we just spent dozens of hours shooting and looting across Pandora in search of the legendary Vault, said to contain unimaginable treasures, and when we get there, a giant fucking monster pops out? And then we kill it, and that's it? There's no treasure, no reward, and no resolution. What did we expect when the planet was called "Pandora"? Shut up, that's what.
How We Would End It: Well, there could have been some awesome weapons behind the giant monster.