Sunshine is my favorite Mario game. Sure, the voice acting is really bad, and sure, the pachinko level exists, but if you just give it a chance, it'll be a hell of an experience! The music is top notch, the atmosphere is amazing, the FLUDDless levels are a nice change of pace, it's just so good. If you only take one thing away from this comment, let it be known that Sunshine is a great game that needs more love. 10/10, would play again.
As someone who hasn't played many of the main series Mario games, I played Super Mario Sunshine to completion and enjoyed it. I don't hold any particular affection for it but it wasn't bad for the most part.
I gave it a chance because when I was growing up I had played Super Mario 64 a few times and enjoyed it. I missed out on the original, famous Mario games because while we had an NES and SNES, I paid more attention to the Sega Genesis and its games and never bothered to try the old Marios. While the F.L.U.D.D. mechanic was definitely different from what I had been expecting after 64, it was intriguing in its own way and I liked exploring Isle Delfino. Overall, I'd probably still take it over something like New Super Mario Bros. U since, not having much experience with the older titles, I'm pretty bad at that sort of platforming.
All this said, I'm more of a fan of the side games for the Mario series. The older Mario Party games, Mario Kart, and some of the Mario sports games have been a lot of fun. I probably should give the Galaxy games and Odyssey a try in the future though.