>>528300609
Unironic answer: it's Summer Skadi in her rerun.
Because the other servants may be powerful, but FGO's gameplay emphatizes way more the busted support over the attacker. Skadi would cover the Quick servants as a whole and allow some of them to loop.
In comparison Aesc/Tonelico and Castoria Summer both need their respective supports to make most use of their power. Whereas the existence of breakbars essentially puts a hard cap in what a servant can do anyway.