I came to realization that there exists Group of "Greatest" & Group of "Strongest"
Oda creates Duos & makes one of them achieves Greatness while other one achieves Title & Power but both are Equals
Greatest are Roger, Shanks, Garp, Luffy, BM (Greatest Empire)
While their Rivals achieves Power & Title & they are WB, Mihawk, Sengoku, Zoro, Kaido
But each Duo are Equals imo
So regarding Garp & Sengoku i believe they are Equals even though Sengoku was Fleet Admiral & have Buddha DF
Just like WB was Strongest Man & had Gura Gura No Mi but still Roger is his Equal ... etc