The most interesting quest?
Hmmm... ... ...
Werewolf island. Definitely.

Yes, that was very fun too. Just liked learning about Balduran Mainly.

I definitly found Durlag's Tower to be the most challenging, but I think that each quest helped me become more proficient at certain skills. The Firewine ruins really honed my skills at ranged attacks(And even moreso, defending against them!!)All I can really say is that I have thoroughly enjoyed ALL of them. I am still playing through for my first time, and have been playing along with my kids, and it's GREAT when we finish a quest together!!!