Jean De Villiers
I think it depends they each have good skills in different things. I mean Nonu is just a straight up beast, he runs fast, tackles hard, and is just a big guy. I think De Villiers is very good though, he is great at setting up plays, he has good evasive skills as well and he is also a great tackler too. It's hard to decide, I'd say Nonu is a better executor and is good at breaking the line alone, and De Villiers is a better playmaker, he sets up the play that can break the line.
I'd rate Nonu higher than de V.