The Elder Scrolls series has had many 'boss battles' throughout its history. Which one was your favorite and why? Here's a few examples (spoilers, obviously)
Morrowind
Oblivion
Skyrim
To Skyrim I'd add Ancano from the Mage's Guild questline and Ulfric Stormcloak / General Tullius from the Civil War questline.
For a pure warrior, the Gaulderson fight was actually quite tough. Having to change tactics based on the brother you were fighting was quite intense, and all three were a significant challenge. Going into that fight without potions or healing magic was essentially suicide, but when I managed to pull it off it felt really rewarding. Challenging in a way that Miraak's healing crap and Harkon's minions never felt.