I'm of the opinion Super Mario Bros. 3 is the most overrated traditional Mario game, after Mario 64. I don't think many people consider Yoshi's Island that great outside of the internet. In fact, when it came out, most people skipped on it to my knowledge, although it DID come out at the end of SNES' life cycle. But that didn't stop a whole slew of my friends being introduced to their first rpg either: Super Mario RPG. I think of the later SNES years, more people played Mario RPG than Yoshi's Island, but this could be purely anecdotal.
My main issue with Mario games is that they tend to have too many levels, most of which are filler and too short. This complaint has never been changed even into adulthood and into the 3d Mario games. This is why YI's the best Mario in my opinion, the platforming, physics and controls aside: it is by far the most polished. It doesn't need to have 10000 levels like Mario World or Mario 3. Every level is unique and doesn't feel regurgitated. They're also not extremely short and allow players many options: you don't HAVE to get every damn ? cloud or stars in YI, but it certainly adds to the game's appeal. Whereas in every Mario since, you need to get x amount of stars to continue, which is bullshit.
The rewarding sections of Sunshine for me were the warp levels. I had more fun with those warp levels the entirety of Mario 64. If Galaxy is nothing bu Sunshine warp levels, and of that quality the entire game without having 9999999 levels just for the sake of padding game length like every Mario game ever, I'll be willing to concede it's on par with or better than YI, but that's a big "if".
Aside from that, I also got great enjoyment from Sunshine's waterpack. Gaining new abilities which would allow to reach new heights, new areas, and achieve new secrets was really appealing. Getting new nozzles gave it a sort of Metroid like feel to the hub world almost, and I liked that.
