The sunshine-filled sequel with a unique sense of identity, it's Super Mario Sunshine!
Development on a sequel for Super Mario 64 went underway various times, but none ever came to fruition, until the summer of August 25, 2002, when Super Mario Sunshine was released. Super Mario Sunshine introduced many new elements, including Yoshi, better controls, and F.L.U.U.D, a mostly-permanent add-on that makes Mario's movement much more interesting. Of course, this game isn't without it's detractors: many have criticized the samey nature of Isle Delfino, the over-abundance of collectibles,etc. Still, it is a game that is worthy of being experienced.
Check out the awesome review by Matthewmatosis below! Note: that review belongs to him, not me.