Invaders! - I think the main reason why I liked it so much was it was such a challenging mission when it was first released....and I always played my Paladin who did fairly well against the beholders. When everyone was running away from them, I was running at them!
Threnal - I loved this quest chain, with the exception of the escort mission with Coyal. If it wasn't for that one mission, I figure more people would run it. Retribution is a fairly decent level 6 item (I want an epic one!)
Giant Caves - It was like playing the lotto each time you ran in there to loot the chests!!!
IQ - It was just a nice refreshing change of pace when it was released. New monsters. Nice shorter missions that one can solo (a first for higher levels). Taking something old and making something new out of it....reversing it to how the first place you land in the game is now one of the last place you go.
Chrono - Excellent way to relive the past event of the tent blowing up!