Dimentio from Super Paper Mario. Entertaining dialogue? Check. Manipulating everyone behind the scenes? Check. Plot to destroy the entire multiverse? Check. The only villain to canonically kill Mario and co.? Check.
Arthas post turning into a Death Knight. Both of them are white haired, green eyed good guys gone bad with mommy/daddy issues, carrying around a legendary weapon, whos final battle is fought against in the snowy north. Let's see someone to that!
Dr Eggman Ridley Dark Samus Ganondorf The Combine advisors/Doctor Breen Glados Agent Edgar Ross The Patriots Bobby Kotick Whoever thought the WiiU was a cool idea Honorable mention: Any villain who isn't Sephiroth.
Saren and the Illusive man. Villains who aren't necessarily evil for the sake of being evil, rather, they believe that what they are doing is for the sake of everyone. Unfortunately, they are misguided due to the indoctrination of the Reapers.
Villains that are better than Sephiroth IMO: Exdeath Kefka Lavos Queen Zeal Ganondorf Majora's Mask Vaati Demise Dr. Wily Sigma Dr. Weil M. Bison Akuma Jedah Dohma Ridley Mother Brain Dark Samus Pokey Minch Giygas Dracula Death Liquid Snake Colonel Volgin Revolver Ocelot SHODAN Kain Andrew Ryan Frank Fontaine Calypso Needles Kane Shang Tsung Shao Kahn The Doctor from Cave Story Ballos Hades from Kid Icarus: Uprising Dr. Eggman