Dwight Yorke by a country mile. I would say he is a moron in the clinical sense. Having met him, and been in his company for about an hour at a social event in Glasgow about 15 years ago, I speak with a level of personal experience on the matter. He is a palpably stupid man.
Don’t agree that Ferdinand is thick. Just because someone isn’t especially articulate doesn’t mean they are unintelligent. Some of the smartest people I’ve dealt with can barely read and write. Think Ferdinand expresses his thoughts in a far clearer and more coherent manner than Yorke, who isn’t capable of independent thought imo.
fwiw, would also say Owen, Beckham and Robson (who didn’t make your list) are discernibly thicker than Ferdinand. Far duller too, especially the first two who are sinfully dull.
Hargreaves, Fletcher and Keane shouldn’t be in there (again imo) as they are relatively bright.