Well...I can't choose but, anyone of the main characters from Batman the animated series, especially Mark Hamill and Keven Conroy.
The entire cast of the Metal gear solid games,
and I must also mention Kevin Michael Richards.
If you hear his voice acting, he usually uses his own dark voice, but in the animated series called The Batman, he plays the Joker very similar to Heath Ledger's potrayal of the character in The Dark Knight.
Richards acctually did the Joker before Ledger (from 2004-2008), so I speculate that Richards version was one of Ledgers key influences.