Mission 22 in DMC1 where you fight Mundus, The first time I played it, I was like this is great gaining Sparda devil-trigger, then I was amazed by the Fiery Dragon he can turn into ( the damage it does ) the fight on the ground using the Red Fireballs and Sparda Sword. I think its Dante's strongest form so far, I don't know why he doesn't use it? maybe he likes a challenge? ( S Rank on all missions except DMD mode )
I was gonna chose DMC3 the last fight with Vergil when he's at his toughest.