I'll say itsuno for DMC3 , but i don't respect him for his recent acts.
DMC3's plot and gameplay + DMC4's gameplay is what finally defined DMC as a franchise after kamiya founded its base, but kamiya should be given his credit for founding the series. Itsuno maintained and improved the gameplay and he added more to the character's personalities. All of that aside from the music and gothic settings.
I respect kamiya more and he's more consistent that itsuno , but itsuno made the "holy grail of HnS games", that is DMC3 ; it expanded kamiya's work on gameplay and characterization.
note: Whether you liked DMC1 dante or anyone, fact is that kamiya made DMC1 dante playful at the beginning and if he expanded on the DMC1's simple-heroic dante, we would be seeing another playful and annoying character as you people claim, like bayonetta. Itsuno expanded on "lets rock baby!" and " Slow down babe!" in DMC3, but introduced the silent and serious type that makes fanboys wet , that is DMC2 dante but he was rejected because it was DMC2 (dat hate).
It was inevitable that DMC1 dante was turning into a playful- over the top character, and in DMC3 he went through things that made him what he is in DMC1, and he had his serious moments in both DMC3 and 4 , that everyone ignores and labels him a drunk or a punk-teenage character , like when credo died and dante said something about his dying wish , then turned serious . He was serious when he assassinated sanctus in DMC4 but fanboyism and DMC4 hate triumphs every fact.