If we're purely talking voice actors, there are SO many incredible talents to choose from: Mark Hamill as The Joker for sure, Kevin Conroy as Batman, Tara Strong as dozens of characters, etc.
Clancy is an incredible actor in his own right both voice and general. I'll always have a soft spot for him as Kurgan in the first Highlander movie. He was so... unsettling.
I don't know. Mark Hamill is a voice acting chameleon. James Earl Jones was himself, which was great for many roles and impossible to improve upon for some. Their strengths lie on different axes.
https://www.imdb.com/name/nm0000317/