Shanks is not a mentor so I'll vote for William despite not knowing who William is.
Don't know much about Gojo or Emiya so I'll random generator that pair.
Voting for Genkai because she's an amazing mentor, not only did she turn Yusuke from fodder into someone strong enough to defeat Toguro, but she was even willing to go so far for her student that she got herself killed in order to make him stronger, not many mentors go that for their students. I also don't know who General Ouki is.
Iroh vs Kakashi, now that's a tough one. Going to vote for Iroh because Iroh was capable of bringing Zuko from the dark side while Kakashi failed to do the same for Sasuke. Sasuke willing to destroy the Leaf village is an L on Kakashi's mentoring skills.