I'll have to say it's a tie between Donald Duck and Scrooge McDuck for me. Donald is very funny as an animated character (especially when the late Clarence Nash voiced him), but he's a very interesting character in comics too, thanks to all those wonderful adventures which were essential to his character development.

Scrooge, however, has a rich backstory, and he's a lot more complex an individual than Donald and quite a tragic figure in some ways. Particularly Carl Barks and Don Rosa developed Scrooge's character into a well-defined character. I also enjoy the animated Scrooge (thanks to the wonderful output of his voice actor Alan Young).