I did love the Maw mission, and think it's one of the best. My favourite T1/G mission is probably the Sword, especially the rectified version with the Little Big World.
Song of the Caverns would my 'best OM architecture' prize.
I liked Mage Towers, but it's the one with the most wasted potential. I'd love to see it redone with better use of empty spaces
Worst missions in my opinion:
T1/G: Thieves Guild
T2: 1st City Bank and Trust. Casing the Joint was nothing special either.
T3: The clocktower mission.