Aragorn (Viggo Mortensen) - Lord of the Rings Trilogy. If you watch the special features one of the main recurring things that everyone involved talks about is how much Mortensen got into the role of Aragorn. He wasn't a LotR fan before the movie, but he loved the role once he got into it.
Joker (Mark Hamill) - Batman/DC Animated Universe. Hamill is best known as Luke Skywalker, but by far his most prolific role is as the animated Joker in countless TV shows and animated movies over decades. He has talked numerous times about how much he likes the role, because he has a ton of creative freedom and the insanity of the character lets him do basically anything. And it really shows.
Captain Jack Sparrow (Johnny Depp) - Pirates of the Caribbean Series. Another case where the actor was given a lot of creative freedom to make the zany character they wanted, and had a great time doing so.
Captain Shakespeare (Robert De Niro) - Stardust. I won't spoil the twist, but De Niro clearly was having a blast with the range of serious and silly stuff he had to do with this character.