Let's see...
ARR Favorite: Hullbreaker NM, Snowcloak, Copperbell Mines HM. Loves the aesthetic and/or fights.

ARR Least: Qarn NM. I enjoy it and the fights but it has the most potential for things to go wrong. Well. That and AV.

HW Favorite: Great Gubal Library HM. Loved running it, great music, fun fights, nice pull lengths, enjoy the aesthetic

HW Least: Hullbreaker HM. Boring, linear, gated, no good fights. This dungeon can go to hell.