This is my ranking of all time captains (excluding Squad Zero)
Top Tier A
1. Sosuke Aizen
2. Genryusai Yamamoto
3. Kisuke Urahara
High Tier B
4. Shunsui Kyoraku
5. Jushiro Ukitake
6. Yoruichi Shihoin
7. Byakuya Kuchiki
8. Kenpachi Zaraki
Solid Tier C
9. Gin Ichimaru
10. Shinji Hirako
11. Mayuri Kurotsuchi
Mid Tier D
12. Soi-Fon
13. Sajin Komamura
14. Kaname Tosen
Low Tier E
15. Kensei Mugurama
16. Rojiro Otoribashi
17. Retsu Unohana
18. Toshiro Hitsugaya
19. Love Akigawa
and just for fun I would add, next in line Captains:
1. Hachigen Ushoda
2. Renji Abari
3. Shuuhei Hisagi
4. Chojiro Sasakibe
5. Rukia Kuchiki/Ikkaku Madarame
Top Tier A
1. Sosuke Aizen
2. Genryusai Yamamoto
3. Kisuke Urahara
High Tier B
4. Shunsui Kyoraku
5. Jushiro Ukitake
6. Yoruichi Shihoin
7. Byakuya Kuchiki
8. Kenpachi Zaraki
Solid Tier C
9. Gin Ichimaru
10. Shinji Hirako
11. Mayuri Kurotsuchi
Mid Tier D
12. Soi-Fon
13. Sajin Komamura
14. Kaname Tosen
Low Tier E
15. Kensei Mugurama
16. Rojiro Otoribashi
17. Retsu Unohana
18. Toshiro Hitsugaya
19. Love Akigawa
and just for fun I would add, next in line Captains:
1. Hachigen Ushoda
2. Renji Abari
3. Shuuhei Hisagi
4. Chojiro Sasakibe
5. Rukia Kuchiki/Ikkaku Madarame