Here's my two cents:
Top Tier: Alpha Patroklos, Natsu
High Tier: Cervantes, Ivy*, Mitsurugi, Omega, Pyrrha
Upper-Mid Tier: Algol*, Astaroth, Lexia, Maxi*, Nightmare, Voldo*, Yoshimitsu*
Mid Tier: Hilde*, Patroklos, Siegfried, Tira, Xiba
Lower-Mid Tier: Aeon, Raphael, Viola
Low Tier: Dampierre, Zwei
* = Not sure
Edit: Gave it some more thought...
Edit 2: More thought after playing PyroZamfir's Siegfried...
I'm sorry, but I still don't understand why Natsu is considered (by most, it seems) to be a top tier character. The only thing she really has in her favour is her high damage - but it is offset by the fact that most of her POS stuff is unsafe and her big combos require counter-hits (which are pretty prevalent in SC, but still). In my limited experience playing as her, she also seems to struggle a lot against Nightmare (and even Siegfried) players who are particularly meticulous about their spacing, as she doesn't seem to have any way to get in. Against Pyrrha, most of her POS setups into a CH combo seem to end up with Natsu eating a stab to the face.
I don't know, maybe someone can enlighten me about this.