For me personally, the Snowman's levle in SM64, mind you, i hadnt played the game in many, many years, probably didnt come past the tiny-small world, i discovered it while randomly jumping about in one of the rooms of the castle, i was quite WTF-ish after i got into the world
Also, in Plok!, for the first time i managed to get into the grand-dad Plok world, which was cool!