Personally, I'd for the Golden Passage. Both are great levels for what they do (one's a tutorial to how the game works, the other a final test of all your gameplay knowledge), but the latter is just a more fun level to play with a lot more of an impressive atmosphere to it. Pity you don't get to play it without the big rush from the Frog Switch, it'd be a rather nice place to explore at a more leisurely pace.