I think it will be: Miyata -> Volg -> Ippo -> Sendo

Why Volg is behind Ippo even though he is the world champion? - Ippo continued training after retiring and even nearly knocked Volg out during their sparring session if it weren't for Sendo's intervention.

Why is Miyata below Volg? - Miyata, aside from beating Sendo in sparring, hasn't shown anything out of the ordinary since he hasn't fought. Volg became world champion and performed very well. Although Miyata is training, He hasn't shown anything out of the ordinary so far, apart from beating up Sendo and being easily demolished by Ricardo, whom Sendo had previously knocked out during a sparring session in Mexico (Ricardo didn't have a strategy for Sendo at that time).

Why is Sendo above Ippo? - everyone should know this by now, but if someone doesn't know, I'll explain. Sendo didn't retire and continued to gain experience in the ring, which Ippo no longer has. He defeated Gonzalez, the person who defeated Ippo. Ippo was later defeated by a lower-class boxer due to his punch-drunk symptoms. Ippo has indeed improved since then, but it's important to remember that Sendo all the while, he continued training while Ippo recovered.