Not to me - and I was collecting Detective and Batman comics in my youth in 1962.
Heath Ledger plays a creepy psychopath, but not the Joker.
The closest to the real character that goes back to 1940 is Jack Nicholson.
Joaquin Phoenix will just be another example of Whollyweird revisionism.
The ultimate fate of comic book characters is to undergo constant reboots and revisions, at least as far as DC and Marvel are concerned.
It’s the nature of the industry, like it or not.
To say that the original Joker is the only authentic Joker is to ignore every single version of the character that has their own merits. Same holds for any other character that’s had a reboot or two or ten (like Batman, or Flash, or Spider-Man, etcetera).