Ledger, Phoenix, Nicholson and Romero all have good performances, but different strengths. Romero and to that regard Nicholson have the cartoony Joker pretty much nailed. Phoenix and Ledger have the more darker and more mental aspects of Joker as strength, but lacking a lot of the more typical cartoon elements, which in a way is a weakness of the portrayals.
If I had to choose who I felt embodied Joker the best, it'd be Ledger. Phoenix had the path of becoming a Joker, but never really functioning as him and it became too much like a commentary on social-political issues. Honestly, loved the movie, but a big part of my own liking of the Joker, is how mythological the character is, making any definitive origin story overall irrelevant in the scheme of things. It's not really the most interesting aspect of him and it often felt like focusing on his own personal issues in a way that wasn't befitting the Joker. Also weak in the comedic aspect.
Nicholson's approach felt a bit overly mob boss-y, though that criticism can be levied against Ledger's as well. Origin is thankfully shorter, which is what I think it should be, heck, the origin story itself should be unreliable, for the best. The comedic aspect is well-done here, although it also being very cartoony, which might be a preference issue. Well done with actually having Joker props, which is something that's missing in Phoenix and Ledger's portrayals for the most part. Still it's solid, but I feel like it really doesn't hit the portrayal just right. (ps: long time since I saw the movie, so had to look up clips to remember and judge)
Romero is the very cartoony Joker, more so even compared to Nicholson. That mustache though, can't ever unsee it.
Ledger overall just convinced me to fear the Joker. Honestly, the criticism I have is that the Joker seemed a bit too in control, lacking the more cowardly or outburst aspect he might show at times. Also, the "grounded" aspect of the portrayal makes a lot of the cartoony aspects, like props and to a degree the comedy, be removed . The psychological aspect is also to a degree less prevalent. But he's the one that overall convinced me the most of him being the Joker.
I'd love to see Mark Hamill take on Joker in a non-cartoon movie. Just to see if he could do it, considering his success voicing the Joker, which he's done very well.
