I vote for Bellatrix, who could not hate her guts? I am not saying that Voldemort, Umbrige and all the other villans are not horrible too. Only that Bellatrix is the most horrible. And I have my reasons, after I state them then I will rank the villans.

1: She Killed Sirius and Laughed about it.
2: She Killed Dobby.
3: This only happens in the movie but She burt Hagrid's house and Jumped for joy.
4: She laughs after Dumbledore dies.
5: She teases Harry.
6: She loves Voldemort.
7: She tourtered Hermione.
8: She threatened to tourture Ginny.
9;She helped kidnap Luna.
10: And so much more.