Easiest: Vergil (For some reason on the 3rd fight, I was able to repeatably attack him during his attack lag without him blocking. I think it was because he was near the corner of the pillars or something. Anyways got an S early on) Favorite: Agni & Rudra (put it on Turbo mode and it's so much fun. Wish they would coordinate more, though) Hardest: Nevan (....... I was distracted.......)
Nevan and Doppelganger. Agni and Rudra are honourable mentions as well, but Nevan and Doppelganger are just so awesome. I mean, Nevan, a succubus-like demoness who turns into a guitar which turn into a scythe! That's just freaking awesome! And Doppelganger gives the coolest Style, imo. Agni and Rudra are just funny. XD
My favourite is the final fight with Vergil; the music and the cutscene before and after the fight just add to the epicness!