DMC1: Overwhelming - Mundus the whole thing, when I first played on hard I used yellow orb when he went into the 2nd part which really took an A from me.
Unoverwhelming - Phandom. He's so pathetic that you can beat him without damage at all

Only in the corridors is a bit dificult but nothing more.
DMC2: Overwhelming - Trismagia the only difficult boss in DMC2. However Dante's escape move make it easier^^
Unoverwhwelming - Well the all Nefasturis/Nefascapitis thing. The only thing that can damage you are those bats, who can't really hurt you
DMC3: Overwhelming - Arkham. The Phantarch I completely agree with what you said. Losing DT and Style in return for Vergil really throws off your rythm. I mean I have battled Arkham in bloody palace (no assistance there) and he was waay easier. Although hard again.
Unoverwhelming - Doppelganger. You all said it. You spoke of Geryon too, but he can be difficult because the mission 12 is really hard in higher levels without using items.