Updated list.
Notes:
Bepo is a placeholder for Joyboy since Joyboy isn't one of the options.
Sabo is above Sanji but not Zoro since, even though Sanji and Zoro are nigh equals, Sanji can't cut through flames like Zoro can, so the matchup is much harder for him.
Added the Tobiroppo.
Added Dorry and Brogy, they don't have many feats so my take on their powerlevels are just pure speculation, take em with a grain of salt
Added Loki and Harald, and I might bump them up to Shanks' level depending on what we see in the future, but for now I'm not going to be that generous, since Loki did still get defeated by Shanks in the story. If we find out that Shanks jumped him or Loki just gets feats that contradict him being below Shanks, I'll bump him up.
Xebec >= Primebeard >= or ~ Roger = Garp = Sengoku = Mihawk (Mihawk could be above or below them, but I think he's at least equal to them since he's above Shanks) = Akainu = Dragon >= Kaido.
Of course, Akainu and Dragon's placements are assuming they fight who I believe they'll fight (Luffy and Warcury), so take that with a grain of salt as well. May not work out like I hope it will.
Ivankov and Prime Kuma are equal even though the list makes it seem like Kuma is slightly above Ivankov.
Last thing, old Gaban being slightly above old Ray but Prime Gaban being slightly below Prime Ray is only because old Gaban seemed to be more active as a fighter so he likely declined a little less than Ray did.