I've done a good number of quests, but haven't got to the Mourning's End quests yet - so they may yet be in the running. Recipe for Disaster is undoubtedly the LONGEST quest there is, but the nice way the subquests are broken up makes it easy to do a few, forget it for awhile, and then do more later. I'm OK with that. The Great Brain Robbery was tough - getting everything to line up, wearing the fish bowl on my head through most of the quest, and fighting the final foe without any prayer assist was just FRUSTRATING, so it would make my top 5 pretty easily. Underground Pass/Regicide was probably the most challenging skill-wise. Find a place to jump. Jump. FALL. Take damage. Walk ALL THE WAY AROUND - go BACK to where you were. Jump. FALL. (you get the idea). Now THAT was a REAL PAIN. I was SOOOO glad when I was done so that I didn't have to go back through that LONG ROAD again. Monkey Madness was fun - putting together the M'Amulet and Monkey Talisman took me awhile, but I enjoyed the challenge. One Small Favor was annoying - you just had to laugh when the next guy in the line asked for ANOTHER favor - but I really wanted the key ring, so it was worth the wait.