Sunshine is the best Mario game in that it has the most sophisticated gameplay and movement mechanics. You have more options for jumping around and manoeuvring than any other mainline Mario game, and these options can be combined with the FLUDD for even more options. That being said, it's the worst because these mechanics are, truly, almost unnecessary to actually beat the game. There are a handful of jumps in Bianco Hills which require Mario to change direction in midair (the FLUDD's biggest addition to the core gameplay), and there are one or two grates in Rico Harbour where you can do the same thing, but even in that case it isn't necessary to get any shine sprites.
You ever watch a speedrun of Sunshine? There's a category called hoverless, which prohibits the use of the hover nozzle; if your game can be completed without the use of one of its iconic gameplay elements, it needs to be rethought.
Most of the jumps and levels in Sunshine are just fuckhueg with a lot of open space, since the FLUDD dramatically extends Mario's ability to cover horizontal space in the air. There are a lot of weird objectives, too, especially in Gelato Beach (I think that's what it's called) and the hotel area: the giant wiggler boss fight is fine, but the watermelon pushing is astonishingly poorly programmed, and the mirror alignment for the sand bird is needlessly cryptic since the game expects you to treat it like a physics-based puzzle all of a sudden.
Mysterious Hotel Delfino is a terrible shine sprite. Even when you know what you're doing it'll take you upwards of ten minutes of pointless running around inside a cramped environment with no interesting jumps and a shitty camera. I don't know about you, but I don't associate cramped indoor environments with fucking Mario platforming. I don't associate long-ass waiting times between trivial jumps, either. Temember getting Yoshi in Delfino Plaza, waiting for the boat, jumping on, waiting for the boat again, jumping off, all for a warp pipe? Temember having to swim the fuck back if you missed either of the jumps? Remember the no-FLUDD level underneath the mushroom village which took minutes to reset?
Honestly, I don't dislike Sunshine. It was my first mainline Mario game and I do remember it fondly, but it doesn't hold a candle to 3D World, let alone something more pure like SM64 or more polished like Galaxy. Sunshine feels more like a bizarre Zelda game in some places than it feels like a Mario game; I enjoy seeing Nintendo try new things and innovate (especially at a time when they were guilty of a lot of the memetic rehashing people accuse them of), but Sunshine needed more time in the oven or a fundamental reworking.
I'm a chuckster!Fuck that level.