It's hard to say for sure, but Tony Stark aka Iron Man from the Marvel universe is often considered very wealthy due to his tech empire and genius inventions.
I think Thurston Howell III from Gilligan's Island is a strong candidate. He always seemed to have endless resources and money at his disposal.
Well, one of the contenders for the richest fictional character could be Scrooge McDuck. His vast fortune is legendary in the cartoon world.