Definitely Kwannon, Destiny, Cypher, Synch, Tempo, Manifold, Greycrow, Nanny and Orphan-Maker for existing characters. Also feel like Polaris has gotten a lot of attention from fans despite not getting to do much in X-Men, people really like what Larraz has done with her and the coffee. And then Apocalypse was already A-list but has definitely been a breakout in a different kind of way.
For new characters, basically all of the Arakii mutants but especially Khora, Isca, and Bei.