After recently blowing through E17, I have to say Titania and Taka.
I think that Taka's just a real sweetheart. Going through the Reshiram route, his dialogue is quite interesting and really depicts his life. He was born into his own hell and listening to how he copes with it, in addition to coming out and aiding your character really makes me feel for the guy.
As for Titania, well, she comes off as an ass, but despite her cynical nature, she really comes off as a caring individual. During the events before the WTC, she spent her time dealing with the Meteor threat by giving them false intel and even shouldering the work of dealing with them to herself. During the events of the WTC, you get to see her selflessness (towards Amaria) despite her own well being. Seeing the events of the Reshiram route, its rather nice to watch her begin to trust the player and begin to start acting for her own well-being (despite resulting in Amaria's breakdown). Her most common phrase is "I feel like shit" and to actually watch her feel good about herself is quite heartwarming.
So overall I just find those two to be the most developed and interesting characters in Reborn.
Before E17, I'd probably take Cal over Taka. I loved his redemtion in Ametrine. As for gym leaders before E16, I'd either say Adrienn and Shelly. Adrienn because I find xys resolve to rebuild Reborn quite admireable. Out of all the gym leaders, I feel like Adrienn is the one that has the greatest difference towards the well-being of the citizens of Reborn. And Shelly cuz I find her adorable and she needs love.
Also why is Terra on the list twice? I understand she's both a gym leader and a Team Meteor member, but why?
Yeah me too. I think this is why we can't have nice things.
True, Cain is an excellent character. He should be on the list. Maybe as "other" or something for characters like him, Anna, and Laura.
EDIT: Oh, he is up. whoops. I vote for Cain now.