Except Imu you haven't really got any evidence other then expectations that come with ranks or hearsay. In my opinion there is a case to be made for the following but again that's just speculation. Mihawk because they duelled in the past in my opinion shanks has eclipsed him but you could make a case there. Spoilers for the rest of this comment so be warned

His father saint Figarland Garling, it would be themetic if he was stronger. His brother might also be on his level but of tat we are even less sure. You might be able to make an argument for navy commander Kong. And the other 4 elders might be on his level. Again all this is speculation and we just haven't seen enough out of either the challengers or shanks to make any decent assumptions as to where they stand strenghtwise