If we're talking greatness in terms of stardom, I think Kucherov is in tough to beat Ovie. The Great 8 is renowned for his flair, flashy goals and power (along with coming into striking distance of catching 99's goals record) which undoubtedly already makes him an icon. Not sure how his legacy is in Russia but I'd assume it's up there also, whereas Kucherov isn't nearly as famous outside hockey circles.
Performance wise, Kucherov could conceivably approach Ovie if 8 retired soon and Kucherov stayed his shifty wizardly self for the next 8 years, since his scoring pace and Cup winning pedigree are his strongest arguments. If the Lightning fall off though, it could affect his personal scoring totals and team accolades and that could lessen his argument for how great his legacy could be.