If you are of the view that peak Kipketer was better than peak Rudisha (despite not being as fast) and was therefore the "best", then how can Rudisha be the "greatest" if he is not also the "best"? Apart from how those terms are being used, it is purely speculative that Kipketer would have been better than Rudisha if they had met - and this while we know that Rudisha had the faster times, plus superior Olympic achievements. Setting aside word definitions, that had Kipketer inhabited the same competitive universe as Rudisha the claim that he might have been faster, while being possible, is incapable of proof. As interesting as the debate is it doesn't really allow for a "correct" answer.