Heath Ledger, by a mile. Jack Nicholson's Joker was less threatening than an actual clown, but that's not to say Nicholson couldn't pull it off, he was just in a cheesy, uninteresting film. Give Jack Torrence of the shining a jokebook and some dodgy makeup and that'd be closer to something that would make a memorable and dangerous villain. Still wouldn't be as good as Heath Ledger, but it'd be close.
And as far as the death thing goes, I honestly think that has probably done more damage to the image of Heath's performance than increasing it. So many people say 'it's just because he died that the performance is so popular'. It isn't, it really was that good. Hell, I'd imagine most nerds didn't give a hoot about Heath Ledger before the Dark Knight was released, you know, when he died. It was only after seeing it that people went, damn, that's a fine actor, damn, he's dead now, that's a shame.
And as far as the death thing goes, I honestly think that has probably done more damage to the image of Heath's performance than increasing it. So many people say 'it's just because he died that the performance is so popular'. It isn't, it really was that good. Hell, I'd imagine most nerds didn't give a hoot about Heath Ledger before the Dark Knight was released, you know, when he died. It was only after seeing it that people went, damn, that's a fine actor, damn, he's dead now, that's a shame.