No. 4 - A Kingdom Divided
The signature quest of the Great Kourend quest series should be listed higher than Experienced, not only for its challenging boss fight but for its intricate storyline involving the hidden corruption and massive conspiracy within the Kourend Council.
But because this list is primarily difficulty-based, we are going to zero in on yet another underrated boss fight. The Xamphur boss encounter is certainly manageable with combat stats that aren’t maxed, but it is still head and shoulders above the majority of the main boss fights from Master quests.
One of his special attacks is literally inspired by one of Vorkath’s signature special attacks, and arguably the most challenging of his attacks at that, from Dragon Slayer II, and that quest is a Grandmaster quest. And let's not forget that one of the Judge of Yama's special attacks is inspired by one of Galvek's attacks from the same quest as well.
Of course, A Kingdom Divided isn’t quite that challenging, but it should still be listed one tier higher than it is.
Xamphur is another boss that should be replayable, though like we pointed out with Sins of the Father's Vanstrom Klause and Taste of Hope's Ranis Drakan, you can technically replay it as many times as you want, within the confines of Quest Speedrunning and the stats/gear you’re provided.