From 3.1: I thought I was going to be happy with Diadem but no, it's completely not what I wanted. By now I'm happy with the MSQ and Vanu Vanu quests, they are very heart-warming and I love Linu.

Whole game: Beautiful and well designed dungeons like Lost City of Amdapor, The Vault and Arboretum, MSQ, beast-tribe quests in general, class quests and Sidequest chains that let you know more about certain NPCs and I love the primal fights, especially Garuda. Also remote places with great design like the Hermit's Hovel.