This was actually really hard. But after some soul-searching and a few walks down memory lane I think I'm pleased with my list.
1. Kathy Bates-Merrick-Tate-Glover
2. Annie Sugden-Kempinski-Brearly
3. Dolly Acaster-Skilbeck (she would be #1 if I only counted the Katharine Barker years)
4. Betty Eagleton
5. Viv Windsor-Hope
6. Sam Pearson
7. Edna Birch
8. Val Lambert-Pollard
9. Amos Brearly
10. Kim Tate-Marchant
11. Laurel Potts-Thomas-Dingle
12. Matthew King
13. Joe Sugden
14. Henry Wilks
15. Jessie Renfrew
16. Matt Skilbeck
17. Caroline Bates
18. Zoe Tate
19. Ashley Thomas
20. Alison Gibbons
(Honourable mentions: Diane Sugden, Bernice Blackstock, Jackie Merrick, Elizabeth Pollard, Frank Tate, Steph Stokes)
I know varying levels of things about the rest of your list, but who is Jessie Renfrew?
Yes, it's Aunt Jessie. I did hesitate a bit about putting her on the list, since she only appeared in a handful of episodes, but she made such a lasting impression that she deserved to be up there.