Between Bianca and Cheren, I'd have to say the latter. Bianca has been rather easy to defeat thusfar. (Granted, I'm not far in the game, but nevertheless) Cheren has been easy, too... but the fact that he's made good use of hold items had made up for it.
As for N, he hasn't been too hard yet, either. So far, I find Cheren has been a tad harder to beat than N, too.