1. Cesar Romero in Batman (1966).
2. Jack Nicholson in Batman (1989).
3. Heath Ledger in The Dark Knight (2008).
4. Jared Leto in Suicide Squad (2016).
5. Joaquin Phoenix in Joker (2019).
Off topic... Just some history ...
The Man Who Laughs (1928)
Conrad Veidt ... Gwynplaine / Lord Clancharlie
Gwynplaine's fixed grin and disturbing clown-like appearance
was a key inspiration for comic book talents
writer Bill Finger and artists Bob Kane and Jerry Robinson
in creating Batman's greatest enemy, The Joker.
May I suggest one small edit? (just adding the article "a" like below)
Whose performance as The Joker in a live action movies or TV series do you think is the best?
I just saw 'Joker' and while I was not very impressed with the film, Phoenix's performance was not only the best portrayal of Joker that I've seen (I've seen Nicholson's and Ledger's) but one of the most exceptional performances that I've ever seen. Wow.