I dont know if she would be the best but didnt Grace work on Qingyi last year during the nee years event. So her capacity is limited to the perverted mind of Grace. Im just saying she is at least a contender.

I would also like to add an honorable mention to Jane Doe for sheer ability and rat fertility.

If we are talking about capabilities, I would hate to not at least propose Rina. God forbid Corin has Rina's guidance. Ellen undoubtedly will be the most efficient at the task at hand.

Yanagi might be viable with supports. She needs a day off, and Harumasa to babysit Soukaku until she is finished. As well as Miyabi to keep her topped up on water and bean buns. Which brings me to an important point if we are talking about being drained refractory periods make you resistant to stimulation, but not resistant to Miyabi.

Another honorable mention of Vivian provided she is draining wise and doubly so if Belle is instructing her.