I'd rank Hisagi above Rukia. He's more rounded off than Rukia. He's stronger and has the mentality which makes him a promising future Captain.
I'd also put Momo above Tetsuzaemon. She's a highly underrated Kido expert.
Renji is a joke. He's only #2 by default because he has Bankai. If Kubo didn't give that to him then he'd be below Hisagi
Rukia and Hisagi are definitely tied, they share lots of personality traits and are equally skilled. I ranked Rukia higher simply because her Shikai is arguably better than Hisagi's.
It's also true that Momo is probably one of the best Kido users, but Tetsuzaemon was quoted in saying that he trained in all forms of Shinigami arts to become a lieutenant, which probably means he's very versatile in combat (which we can't really confirm until Kubo gives him more screen time...), that's why I placed him in the middle. Until we see Momo fight at full strength her place on the list is pretty solid.
And yeah, Renji is stupidly overrated. Without his Bankai he'd probably only be above Omaeda.
I hate to say this because I will get flame and this is just a theory but yachiru is def captain level. Enormous reiatsu and I guarantee she has a Shikai I say this because she random talks and it would leave me to believe that she always has convo with her zanpakouto.
Just a theory though and I said captain level because I believe she possesses that much reiatsu.