Akhaten

Akhaten was the main antagonist of the Doctor Who story, "The Rings of Akhaten" and is one of the more powerful and truly cosmic monsters the Doctor has faced : being a parasitic, living planet that posed as a false God to numerous worlds surrounding itself and threatened to destroy them until the Doctor managed to confront the monster and destroyed it once and for all using his own memories as a Time Lord to overfeed the parasite to the point it imploded in on itself.

Trivia
Akhaten is similar to Ego the Living Planet from Marvel but has a unique, parasitic feel - it is also one of the Doctor's largest enemies, since it is literally a planet-sized entity (though the Doctor has faced other massive enemies, due to his immense number of cosmic adventures across near-infinite realities)