I would like to nominate Super Mario World 2 Plus 3: The Essence Star. It's a Super Mario World hack and the best I've ever played thus far (well, I haven't played that many, but still). A great and intriguing story, an RPG-like hub, non-linear overworld and levels, incredibly fun theme-focused level design, and just a general really authentic feel. I'm playing through the game right now, and I've just about reached the end.