I'd say Nero, the Devil Bringer was interesting, but switching between Styles as Dante was cool too.
I can't even vote, because I liked playing with both of them, equally.
That...that came out wrong. You know what I mean; likewise, being able to switch between Dante's styles on the fly while switching between weapons kept things interesting, allowing the player to try out a whole myriad of different combos, while also getting to test out which weapons worked best on what enemies.
Nero was good, because DMC4 was the first game I played in the DMC series (I lacked the console for the previous games, before; I bought the HD Collection shortly after getting DMC4). As such, I felt he was a good way to ease newcomers into the franchise, but at the same time (and I know The Gaming Brit said this, but I agree with him) he still provides just enough of a challenge to keep things interesting. He may only have three weapons at his command, all of which can be utilized at the same time, more or less, but he still has a fair bit to offer.
Once I got a good feel for both of them, I had no trouble going back and forth, depending on what level I felt like messing around in.
That, and I love both of their snark. I never, never get tired of it.