I admit that I am biased, Twilight Princess being my first Zelda game and one of my favourites, but I absolutely adore the Arbiter's Grounds. Normally I'm not a fan of sand-themed dungeons, but I always look forward to playing through the Arbiter's Grounds whenever I replay Twilight Princess. The buildup to the dungeon is awesome, and the atmosphere inside the dungeon is fantastic. Dimly lit rooms, sand temple-y music, undead enemies, it's perfectly fitting for an old prison located in a desert. The miniboss is awesome, and the dungeon item is one of the coolest in the game (at least inside of that dungeon), which makes the second half of the dungeon loads of fun. Then there's Stallord, the boss. While far from the most difficult boss in the game, it's fought in a very unique manner, using the Spinner to render it vulnerable to your attacks.
The game tells you so little about the place too, and I love speculating. There are a lot of hints and few facts, and many questions to ponder as you make your way through the dungeon. It's not the most challenging dungeon, but it's both unique, memorable and enjoyable in my opinion, and has an awesome dungeon item, mini-boss and actual boss.
In short, the Arbiter's Ground has you playing pinball with a giant skeleton dragon's spine and a horde of undead soldiers while surfing around on an ancient Beyblade.