In order from highest to least highest: Greb Langford Armstrong Walker Duran Pacquiao There's no need to specify where exactly you'd rank them on a complete list of ATGs (though feel free to provide that info if you've ever made such a list)... just rank them in comparison to one another. Myself, I rank them in the order that you see above. I consider Greb to be one of the 2 best fighters of all time (alongside SRR), Langford just behind them as the 3rd best, Armstrong around 4-5 (interchangeable w/ Charles), Walker #6, Duran #8 (one spot behind Ali), & Pacquiao #9.
Langford Greb Duran Armstrong Pacquiao Walker More complete list: 1. Sugar Ray Robinson This content is protected This content is protected 4. Muhammad Ali This content is protected 7. Bob Fitzsimmons 8. Willie Pep 9. Joe Louis 10. Ezzard Charles 11. Roy Jones Junior 12. Joe Gans 13. Sugar Ray Leonard 14. Floyd Mayweather This content is protected 16. Archie Moore This content is protected 18. Pernell Whittaker 19. Packy McFarland 20. Rocky Marciano 21. Barney Ross 22. Marvin Hagler 23. Jimmy Wilde 24. Tommy Hearns 25. Gene Tunney
1. SRR 2. GREB 3. LANGFORD 4. ARMSTRONG 5. Charles 6. Fitzsimmons 7. DURAN 8. B. Leonard 9. Louis 10. Ali 11. Pep 12. WALKER 13. Gans 14. McFarland 15. Ross 16. Wilde 17. SRL 18. Mayweather 19. PACQUIAO
Your top 12 isn't all that dissimilar from mine, except I would swap the general places of Walker & Fitz w/ one another.