TVs a hard one, if we were saying pantomime villains then Mr Mackay from Porridge is your clear winner, what a character. Truly evil villains we're talking DCI Jim Keats in Ashes to Ashes the slimy git. And then there's the Shredder, Oroku Saki, what a heroic villain.
But for film look no further than Joan Severance's character Eve in the 1989 classic, See No Evil, Hear No Evil. Probably the sexiest villain of all time. Honourable mentions to White Goodman of Dodgeball fame and the Trunchbull from Matilda.
TV Series - Slade Wilson/Deathstroke from Arrow although Joffrey Baratheon gives him a great run for it, Jack Gleeson did a great job in making a villain who was pretty much universally hated.
Film - Heath Ledger's Joker, also really liked the fan theory that the Joker was ex military, given his expertise with weaponary and his level of tactical precision.
Found this incarnation of Moriarty disappointing tbh.
Gene Hackman was always a good bastard in films.
His Lex Luthor is more rememorable and charismatic than others :
And he plays a right cruel bastard both in The Quick And The Dead...
... and in Unforgiven
He's also a very good 'goodie'; one of those rare actors thats comfortable in both roles. With others it just doesn't sit right, when for instance Tom Cruise or Tom Hanks play villains, but Hackman can move between roles like that with ease.