Definitely the Aquatic Ruins. It's the perfect blend of water and land in a Sonic stage, not to mention it's nicely tucked away at the feet of the mountains. There's a few nifty things I like about it as well. Some of the plant life hovers so low to the ground that Sonic rustles through the leafs causing some of them fall. Also, there's waterfalls, loops, hollowed logs, and Grecian columns all over the place. In some places, Sonic can even skid across the lake surface. Did I mention the AWESOME soundtrack for this level?