Cesar Romero

I don't know what this means.

If you mean what actor's portrayal was the closest to the character in the comics, then Cesar Romero is a pretty good candidate.  He was pretty spot-on, for that time period.  The Jack Nicholson version had elements of the 1940s debut of the Joker, as a random murderer, but the scripted version was based on the Englehart and Rogers use of him, in "The Laughing Fish," plus other elements in the whole run, like Boss Rupert Thorne, as a political mover and shaker, who is also the head of crime operations in Gotham, Vicky Vale is patterned more after Silver St Cloud, the love interest, than she was the comics Vale, who was an old character, designed to be a Batman version of Lois Lane (Kane claimed based on Marilyn Monroe, who he claimed to have met before she was famous, to which I cry BS, which is my go-to response for a large portion of what Kane claimed, in interviews).

Heath Ledger was doing something different as the Joker was written to be more a representative of random violence, using him as a metaphor for things like the 9/11 attacks, or mass shooters, or other catastrophic, violent events, which seem random, until you investigate the backgrounds of the perpetrators and discover that they were very calculated.  The comics started turning the Joker more into that, as a response to the popularity of the film, rather than vice versa.  That random element was there, to a certain extent, from time to time, but not quite in the same manner as Ledger's version.  Maybe the Joker's debut story, more than most.  Later stories would have what seemed to be random attacks, but there was always a pattern that Batman would discern.  Then, they turned him into Hannibal Lector.

I didn't see the Joaquin Phoenix thing and have no desire to do so.  What I saw in the trailer seemed like someone read The Killing Joke, then hacked it up, threw the script into the air, and picked up pieces and taped them together.  Then again, a lot of trailers look like that, anymore.

Quite frankly, the best portrayal, overall, was Mark Hammill, in Batman TAS and the scripted character captured the best elements from across the decades.

I've always thought Tim Burton should've cast Jeff Goldblum instead of Jack Nicholson in the '89 movie. He was a closer match to the comics version physically and I think he could've delivered a truly unhinged Joker.

Cei-U!
I summon the might-have-beens!

Thank you! For I moment I thought he meant "Who is the closest person to the joker?", but he said "what". So, the closest object?

A couple of times, I printed black-and-white copies of images of film personalities that I thought would be good as the Joker, and then I colored the lips red and the hair green. I did it for Harpo Marx and Edward G. Robinson.

No lie, the first thought that popped into my head before a scrolled down to see the rest of your post was: "No way, that's Harvey Dent!"