wasnt it "first foe and perhaps greatest rival" or something like that? And don't forget the rivalry between Roger and WB is a more friendly one. There is no doubt imo that Xebec had been the strongest pirate and stronger than Roger until God Valley but that's where Roger surpassed him and become the greatest pirate. If Oda comes up with something else basicially Roger would be a fraud.
Mihawk himself stated that the greatest power on the sea is the ability to get others to fight for you. I’m sure Roger could do that as well. Having the marines and HKs all fighting with him against Rox pirates would showcase that.