Lost City of Amdapor:
interesting boss mechanic, unique mobs, overall design and music

Halatali HM:
nice variation of mobs, ability to use the environment to your advantage (falling morningstars), music, the hillarious positions during "Standstill" and the hate towards that Lalafel for casting it nonstop

Sunken Temple of Quarn:
interesting mechanics and boss fights, puzzles