I suppose I'll write up my personal thoughts now. Was going to wait a bit longer so as not to bias anyone reading, but I don't want to forget.
If we exclude Alexander from the discussion (otherwise he wins for me overall, given that I really enjoy both tanks and mage knights, and also, he's definitely deliberately very good as the required use character), I think the best is Ariadne. She's my go-to for destroying large packs with spread Burning stacks, but she also has good focus damage with Incinerate, and I think her overall kit is just very nicely cohesive.
Ironically, Ligaea and Calysia are probably tied for second strongest to me. Ligaea can just delete enemies if properly setup, and overall her single target output is just extremely high. Since the game definitely has a lot of priority targets to kill, that has a lot of utility. Honestly, it's only how many things resist ranged physical that keeps her from breaking the game. XD Especially now that she has Barrage. But she has a very high skill floor for effective use, so I see why it would be easy to feel like she's weak. It's very in character for her, though, haha. As for Calysia, her single target output is actually quite strong as well, especially for someone who's mainly a healer. If you need heals, she gives you heals. XD Lorena's healing is workable, but definitely not on the same level. But mainly, I think she's very strong in that she does her main job very well while having decently good ability to do other things too.
Weakest really is hard for me to say. Before the recent rework, definitely would've been Sindarion. After the rework, I lean toward Cynthia by a tiny margin, mainly because there can be some awkwardness in properly building/using Fury for her (something I want to adjust in the future). Plus, while she's extremely tanky if played defensively, that does cripple her damage output, while her damage is pretty high in Blood Oath but at the cost of putting her at dangerously low HP sometimes. None of that makes her *bad*, but I do feel like she's the least good by a tiny margin, at least for how I tend to play.