Ridiculous question. They're all on different "universes" with absurdly different "power scales". Glados is the best antagonist of the bunch BY FAR, but none of them can even hold a candle to Sephiroth given how absurdly overpowered he is compared to everyone else's power. Yeah, the Lich King is a massive titan and extremely powerful, yattayattayatta... It's irrelevant when Sephiroth can cut through an entire skyscrapper with a single hand without even breaking a sweat, while moving at the speed of a fucking jet fighter...
Worse, it's just pointless. They all exist in different universes for a reason.