if it's because of quality of opposition and number of people faced i would put holyfield first. if it's because of quality of opposition no matter age, that is, names, i would put pacquiao first. if it's because i say it because i have to be honest with what i think then:

1 mayweather
2 vitali
3 pacman
4 marquez
5 ward
6 martinez
7 wlad
8 donaire
9 bradley
10 khan