You are not sure about Davydenko? And you put JCF in front of Del Potro? Great stuff mate, just awesome! And if you are refering to current form only, then both Potro and Kolya should be out cause they're not playing at the moment.
And what are del porto's major achievements on clay...one FO semi? He's a little overrated on clay imo.
As for davydenko, I haven't seen him play well on clay in some time, so that's why I'm not sure (same for del potro really).