my favourite quest would be the one where you fight the rakk hive. the reason why is beacuse from the point on i would just look at all of the hives on the ground and just expect them to rise out of the ground and step on em. Also it was fun tearing through all my ammo to see it fall with a thud on the ground.
You. Will. Die. It's fun to invite noobies to help you with the quest but instead they start screaming for their lives with a 10-ton Crabworm chasing them and spitting purple acid balls at them. Haha, its priceless.
I liked the quests off of Knoxx's DLC. Especially the long travel to the armory and final boss fight, You.Will.Die was epic too! I even loved the Circle of Duty, even though many people hated it.
I didn't really dig the Knoxx DLC too much but I had a lot of fun playing Dead Haven. Best quest for me by far.