As far as intro stages goes, this one is my fav, mainly for the natural scenery and the fact it's a castle, and you don't encounter Castle themed stages too often in Sonic Games.
After wrecking the Egg Station, Sonic and his bro venture to the Region close to Never Lake by plane, arriving at the ruins of a lakeside medieval castle. A fusion between Sonic 2's Aquatic Ruins and Sonic 3's Marble Garden, the duel run and swim through Sylvania's flooded corridors and scale it's high towers from dusk to night, running into the Mad Doctor along the way who plans on using little Planet to power a new Death Egg he's in progress of constructing.
One thing ya gotta dig here is the Castlevania-ish music, especially Act 2 and 3, sounds like Aquarius, Mad Forest and other Castlevania tunes I'm all too familiar with.