I mean this is Shounen, I think when Zoro decided that his ambition is to become strongest swordsman in the world, basic shounen troupe suggests that he will be strongest swordsman ever.
Although I never bought Roger is level above current top tiers nonsense anyways, there he is teaming up with Garp to defeat Rox. Honestly think both Whitebeard and Kaido are stronger than Roger.
EoS Zoro will proabably be stronger than Roger as well and Luffy might be a whole tier above Roger.