Disclaimer: These are only from anime that I have watched personally (either partially or completely). I don't keep very good track of it sometimes, but I have seen all mainstream titles in anime that I am aware of, and quite a bit of less popular or fringe shows. Though admittedly I am biased towards Action, Adventure, Sci-Fi, Horror, and Mystery shows.
That said here is my top 25 list of Mentors in animes I have seen. I gave these rankings based on my perceived effectiveness of their mentorship on their pupil, their resources and methods of training if any, their inspiration to their pupil and their outlook on the world, their own strength in respect to the given anime world, and their personality (subjective to my tastes of course).
In order of Greatest to Least:
1. Biscuit Kreuger - Hunter x Hunter
2. All Might - My Hero Academy
3. Izumi Curtis - Full Metal Alchemist
4. Victor Nikiforov - Yuri on Ice
5. Yato - Noragami
6. Reigen Arataka - Mob Psycho 100
7. Jiraiya - Naruto
8. Silvers Rayleigh - One Piece
9. Kisuke Urahara - Bleach
10. Kamina - Gurren Lagann
11. Baron Zeppeli - JoJo's Bizarre Adventure
12. Red Haired Shanks - One Piece
13. Saitama - One Punch Man
14. Ur - Fairy Tail
15. Hiko Seijūrō XIII - Rouroni Kenshin
16. Reborn - Katekyo Hitman Reborn
17. Genkai - YuYu Hakusho
18. Master Roshi - Dragon Ball
19. Emperor Ivankov - One Piece
20. Eikichi Onizuka - Great Teacher Onizuka
21. Bulat - Akame Ga Kill
22. Sindbad - Magi
23. Picollo - Dragon Ball Z
24. Cross - D. Gray Man
25. Franken Stein - Soul Eater