the admirals duke it out‼

I would argue that Aokiji is more like Garp than Aikanu and Sengoku sees that as someone he can count on; I think Sengoku and Garp look up to each other more than they do to themselves. Something like that.

Well, tbh, Garp should've been an Admiral by now.
He would likely to have been chosen by Sengoku as his replacement.

But amongst the 3, I do think that Aokiji has the most versatile power.
His power can be used not only for destructive, but also for strategic and defensive purposes.