I don't think there's any total agreement on it, and to a degree it depends on your familiarity with the characters, since some of them can be very hard to use at first but are actually very powerful when learned. Youmu is probably the best example of that. But anyway, my opinion on it:
Top tier: Medicine, Aya
High tier: Marisa, Eiki, Cirno, Reimu, Youmu
Mid tier: Komachi, Lyrica, Reisen, Mystia,
Low tier: Yuuka, Tewi
Bottom tier: Sakuya
Merlin and Lunasa aren't playable in story mode, but just for fighting the AI in match mode, Lunasa is pretty strong and Merlin is probably the absolute worst.
Also, for Extra mode things would be rather different, so don't assume it's the same. Aya is terrible in Extra, for instance.