I have a few favorites:
Plane of Hate. The atmosphere is simply too perfect. It's as if the entire plane is saying "I hate you"
Paludal Caverns. Okay, people are mean, the trains are absurd, but I've had a great time there. Forget the recondites, ever get a group simply to explore the Shik' Nar tunnels and take on the Giant Shroom? It was an adventure.
Plane of Sky. Alright, it's nothing too special, but I love how you exit it:)
Plane of Tranquility. Same as PoH, atmosphere. plus, it's the gateway to most Planes, so it gets extra points.