According to Japanese 12.3 rating site, http://tenco.info/ the top tier is Yuyuko, Suika, Okuu, and Iku. Bottom tier is Cirno and Meiling, with Suwako, Alice, and Sakuya being slightly less bad. However, there are still players who can destroy even with the bottom tier, so it seems like a decently balanced game, at least for a doujin fighter.
Correct me if I'm wrong, but is Tenco really considered the reliable reference for tiers? Urahiso has one of its own, and supposedly its creators discount Tenco because they expect a certain amount of skill (though I don't know enough to have an opinion of one over the other). Urahiso's order goes like this:
S: Iku, Remilia, Sakuya, Yuyuko
A: Patchouli, Reisen, Yukari
B: Tenshi, Marisa, Suika, Youmu, Okuu
B-: Reimu, China
C: Suwako, Komachi, Aya, Sanae
D: Cirno, Alice
Yeah, apparently Sakuya is top-tier. Individual player skill (and their tourney success) certainly would affect how good a character appears. For instance, I believe KamiG (a Marisa player) placed second in this big tourney recently, losing to a Sakuya.
Again, I don't know enough about all this to state which one is more reliable, but I'm putting this here for reference/discussion.