Probably Kobaryo - Tool Assisted Speedcore and Kobaryo - Bookmaker, but osu! was really my introduction to the whole genre, so there's a lot of songs I heard from osu! first
I haven't heard of Xi, Yooh or Demetori before osu! and now I can't get enough. Ice Angel is particularly a favorite of mine from Yooh, and I can get behind just about any song from Demetori -- The MadPiero Laughs, Deterministic Chaos, and Emotional Skyscraper are my favs though!
Other artists osu! put me onto has been Igorrr and Sewerslvt.