Mario Sunshine is an interesting one: it has a lot of good and a lot of bad.
The Fludd-less levels are amazing, and were really representative of what a 3D Mario in the classic formula would be like. The graphics were also very impressive, with some fantastic water effects and a good attention to detail. The game also had much more fluid controls than Mario 64, and running around in the hub world felt fantastic. Nice atmosphere.
But the other hand, the story and voice acting were horrid. The whole "Mama Peach" thing is just absolutely stupid, everything with Bowser was terrible and FLUDD itself was extremely annoying. The hover nozzle over simplifying things wasn't fun - there's no way the Super Guide compares to it, since the Super Guide appears only after numerous deaths and can be chosen by less experienced players if they so desire. The hover nozzle is available at all times, and trivializes puzzles. Blue coins are horrible; there's a reason they never came back. I also didn't like how the only real objective in the game was to finish all of those really dumb "Spray Shadow Mario until he disappears" mission, but that's just personal opinion. And then that just leads to one of the most poorly designed final levels I've played.
Overall, Mario Sunshine is easily the worst mainline Mario game, but it's still probably the best platformer from last generation. Mario Galaxy came in and showed how to make a real quality Mario game, though. I would've loved to see a full game like those Fludd-less levels, with more fleshed out ideas.