Ulquiorra and Saber.?

Definitely Saber's Excalibur. Saber's Noble Phantasm has proven powerful and reliable in a pinch. The thing defeated Ea; World Breaker, Caldbolg and Berserker's gigantic mace to boot, despite the fact that Berserker imbued it with his strength plus a million tons worth of power. She can definitely own anything and anyone, being the most powerful Servant ever summoned.