I rank them more in tiers than actually by numbers...

Tier I (The clear Top 2):
1. Sugar Ray Robinson
2. Muhammad Ali

Tier II:
3/4. Joe Louis and Henry Armstrong (in either order)
5/6. Roberto Duran and Willie Pep (in either order)

Tier III (no particular order)
Sugar Ray Leonard, Pernell Whitaker, Carlos Monzon, Benny Leonard