Frigid Outskirts for daaaaaaaaaaaaays
Shrine of Amana was fun in retrospect since it was a pure challenge optimization question of when to switch magic vs physical defense, and yes I played it as a melee character pre-patch.
Lost Izalith was more un-finished than anything, and the whining about BoC was always over the top. It's a short run back, a unique concept, and the damage done to it lasts through death.
Both Blighttown and Black Gulch have the players peeling back their oppressive layers, with fun little "side challenge" things like the torches or the plague shooter gauntlet for the firekeeper soul.
4-2 combines necromancer challenge rooms with melee gauntlets which yes would be less irritating without the stingrays, but aren't that bad.
Iron Passage was, like Amana, pure challenge run and knowing what to focus on and when. Plus, iirc, there was that sprint section at the start that could open a new area? That was neat.
Frigid Outskirts, on the other hand, has awful enemies, basically no level design whatsoever beyond "you can't see anything", and just doubled up on a boss copy from the same level. There's no way to interpret any aspect of this area as good. Jesus Christ.