Vergil was definitely the most difficult first time round, there's little explanation as to why You need to be Devil Triggered to finish him.
Mundus Spawn is also frustrating because it's pattern is odd and it's hitboxes are all over the place, sloppy boss to be honest.
Bob Barbas, while not exactly challenging, is the most visually impressive I found, the moment I pulled Myself into Aerial News-Coptor view, My mind was in My lap.