My communities are primarily Fate (Nasuverse), Harry Potter, and RWBY, because RWBY fanfic is somehow better than canon.

Fun fact: The Divine Comedy (Inferno, Purgatorio, Paradisio) by Dante Alighieri is probably the first ever self-insert fanfic (dating to 1321), being a fanfic of the Bible where Dante, as himself, gets to meet a slew of historical figures and eventually God. In this SI fanfic, Dante gets to be mentored by his personal hero and fellow fanfic author, Virgil, who is famous for the Aenid (published 19 BC), which is his fanfic of the Iliad.