Interesting vid and discussion. In some ways I think they are very much incomparable as Knox said
Romero, Nicholson, Hamill, Ledger and now Phoenix. Everyone has put their own unique spin on the character and it has always produced wonderful results (and, yes, I purposely left Leto off the list as we don't about him).
Each plays up a different potential aspect of the character, as after all to try and play all of them in a film especially would probably not work for story character cohesiveness in the alloted 120 or so minutes, even the much maligned Leto, if you take it as a very psychotic gangster did fairly well and is not dissimilar to Jack Nicholson, a former gangster turned made by falling into the chemicals. No performance has been 'bad' just different to date in my view, Ledger was the irresistible force embodied, no real origin no need of one, trying to show everyone that thy were, deep down, just like him in some dark way, Phoenix the man who finally cracked as the pressures and neglect of society and family crushed him, a Joker that needed an origin, so hard to compare the two, both would've loved to see more of though.