Spoilers typed below, some taken from the discussion topic. I really can't discuss some stuff here without spoilers, so...be warned.
As for the 4 (5) big bosses getting support (or no support):
1) Mitsuo Kubo (Shadow Mitsuo, 8-bit Hero, whatever): So...really don't think we'd be ranking him - just the shadow form. They really aren't the same person (not that I'll object too much, but...). Mitsuo denies the shadow, much like the other characters do, causing the shadow form to be born. It's separate from him, similar to the other shadow forms (different voice), especially due to the fact that Mitsuo doesn't accept it (still denies it), unlike the other characters, so, plot-wise, it is still separate from him. Now, if you just want to rank the Shadow Form, that's a different story, just noting that the Shadow Form...isn't the person, for whatever that's worth.
2) Taro Namatame (Kunino-Sagiri): I think I explained this a while ago somewhere to someone. From what I remember reading on interpretations, the plot in him is as follows. Kunino-Sagiri is the Goddess of the Fog (Ameno being the God). Taro was given the power of persona and the ability to enter the TV from Izanami. For the battle, it is him - while he doesn't fight like Adachi does, it is him (voice, attitude, he's drained after the battle, hence why he's put in the hospital afterwards). He merges with his Shadow (i.e., as the game plot puts it, his persona) to become Hippie Jesus (i.e., the peaceful saviour). He's fully in control of it, and as the game states, those given the power can mold it however they want to achieve what they believe is the true answer.
3) Tohru Adachi (The Killer): Really doesn't need much explanation. Only issue here is the spoilerness (which, due to the deal made about it...makes it a bigger spoiler than it would have been :/). Do note that he states he is entirely in control of his actions, though, which matters when we get to...
4) Ameno-Sagiri (Big Fucking Eye): A being Izanami created secondary to Adachi's motivations (she was watching to see how a few random people with power would use it - since Adachi was first to enter the town and make use of it, she put him at the forefront of everything, giving form to Ameno-Sagiri to create the world Adachi was aiming for). Once Adachi is beaten, Ameno-Sagiri recognizes this - others with similar power are trying to change the original paradigm, and so he tests whether the new aims of the party can overcome him (i.e., those of the original "creator", as it were). Of all the bosses (excluding Mitsuo), Ameno is the most Necron-like (not that he really is, but most like), and has the shortest exposition (and requires some digging and thought), so probably the least important to rank, to me, on a plot aspect.
5) Izanami (The Goddess): See a lot of the above. She comes out near the end, but is technically the 4th character you meet in the game (Soji - Nanako - Ryotaro - Izanami in Drag). If you talk to people often, you get this from her as the Gas Station Attendant, and she's alluded to many times (and, if you look closely at the Fog Battle at the start of the game, you can make out the Gas Station Attendant Outline - she actually reminds me of Beatrice a lot). She is the orchestrator of the game, though is a little obscure, but honestly, I think everyone fights her, and knows about her existence. Definitely support ranking her - she's arguably the least spoileriffic of all the above, so would be fine to rank now.
Just wanted to put those notes out there. Considering changing my ranking thoughts, but will wait a bit.