I'd have to go with Final Destination. The level has no platforms, no environmental hazards... nothing but you, 3 friends, and the ground. It's the ultimate in simplicity in the way of SSB levels. I like simplicity - puts more focus on your actual fighting skills and less on your ability to stay away until you can kill.
My favorite level is Cornia because it still has the simplicity to it and it's a little cooler looking then Final Destination, well not the background, but the actual stage is.
Final Destination is definetly a good way to test your skills, but I would have to say my favorite is Shadow Moses Island, just because of the overall mysterious feel to the level.
You can discuss this further in the main thread for SSBB. There is no real reason for a thread just for this (especially when some people don't think it is worth it to write an explanation, and thus are spamming.)
