As the title asks, what is your favorite dungeon/trial/arena? Factors can include the difficulty, theme/artwork, story, etc.
Mine would have to be City of Ash II. The Elder Scrolls IV: Oblivion was my first game in the franchise, so that dungeon in particular gives me nostalgia fighting through Oblivion's lava and enemies. Also, the Ash Titan and Valkyn Skoria are two of my favorite bosses.