Her performance on American Horror Story "Asylum"
Lily was amazing as Sister Mary Eunice on S2 "Asylum" or American Horror Story. i think she even outshined Jessica Lange, which was no small feat. The transition she made from the meek nun to the evil one was fantastic. I don't think anything she has done since has matched that, though I do like her in just about anything.
https://www.imdb.com/name/nm1143816/mediaviewer/rm2032795136?ref_=nmmi_mi_tt_sf_6