The Joker has been portrayed by numerous actors since the 1960s, but if you grew up watching Batman: The Animated Series, chances are Mark Hamill will always be your favorite version of the DC villain. Since 1992, Hamill has voiced the Clown Prince of Crime on and off in television, video games and even the Batman: The Killing Joke animated movie. Although Hamill has never played The Joker in live action (though he'll be playing someone similar-looking on The Flash soon), that hasn't stopped him from enjoying the different cinematic portrayals, including, most recently, Jared Leto's version in Suicide Squad.
When asked by Polygon what he though of Leto's Joker performance in Suicide Squad, Mark Hamill revealed that he loved the performance, while also noting that he hasn't yet seen an interpretation of the cackling antagonist yet that he's considered "terrible." His perspective is that every version of the Joker needs to be different depending on the tale being told. In his words:
Everyone brings a different spin to the character. I don't think there's a definitive version of the Joker and I don't think there can be. It's like Hamlet, really. It'll be constantly redefined.
Since being introduced in 1940, The Joker has arguably become the most famous comic book villain of all time, and each actor and creative mind has taken a different approach with adapting him. Jared Leto's Joker in Suicide Squad was definitely among the weirder interpretations, namely with the tattoos and bejeweled teeth, but Mark Hamill believes there's always room for redefining, as long as the spirit and basic elements of the characters remain intact. Chalk-white skin, green hair, red lips, experiencing joy from others suffering, etc.
If anyone is qualified to give an opinion of The Joker, it's Hamill. Aside from voicing the character for over 20 years, the version he played in the DC Animated Universe (seen above) was definitely the most balanced of the versions seen outside the comic books. Whether he was being used more as a goofy prankster or a psychopathic killer, he always had that comical edge to him. The Joker is a psychotic freak, but when he's hurting you, he'll do it with a smile and an amusing quip. As for Suicide Squad's Joker, Jared Leto's interpretation earned mixed reception, and while many will agree that the actor went overboard with his method approach, at least his take on the villain was unique compared to past iterations.
There's no official word yet on if/when Jared Leto will reprise Joker, though it'd be wise to keep an eye on that solo Batman movie or the Harley Quinn/Birds of Prey spinoff as possible places to return. As for Mark Hamill, he's currently voicing the Clown Prince of Crime on Cartoon Network's Justice League Action.