Well, like many before me, I love the DCAU's version of the Joker. It's the perfect blend of manic, whimsy and derangement, especially in later seasons. It was the perfect dramatic foil to Batman's perpetual frown. Put simply, Joker played his role with style, aided by the sheer infectiousness of Mark Hamill's laugh.
In a similar vein: The Music Meister. Neil Patrick Harris does wonders with that role.
Can't find a youtube clip of the scene itself, but here's the music accompanying his debut:
Though really, you need the scene itself to fully appeciate it.
Azula in Avatar: The Last Airbender gets points for being a cold calculating villain, and who actually remains a threat up until she snaps (after which she's arguably more dangerous simply by the way that she's thrown planning out the window, instead acting almost purely on instinct...which is horrifying in an incredibly powerful character who typically could play the rest of the cast like fiddles).
Then we have Frollo, who's on the list partially due to the way that in the face of his actions, which we as the audience generally view as unquestionably evil, or at least horrible, he still maintains that his actions are righteous, even to the point of gleefully reciting "And he shall smite the wicked and plunge them into the fiery pit!" as he prepares to murder the protagonists. And then of course there's his song Hellfire...