i really enjoy wario's woods, but it's pretty obscure and does not get as much love as it deserves.

C'mon, I think we all know Luigi deserves some recognition when it comes to 'underrated'.

There's multitudes of good spin-offs, so I'll mostly stick to the main Mario series and keep it short:

Most underrated 2D Mario IMO is probably Super Mario Land 2 because I feel that title needs to be talked about more. Very good looking visuals, nice music, and a structure like no other 2D Mario before or after. Instead of doing things in a mostly set order, the overworld right from the start lets you choose any of the six main worlds ala Mega Man. And those worlds deviate from the standard grassy plains, desert, lava world, ect. from most other Mario games with neat locales such as a cemetery, outer space, or a giant mechanical Mario full of toys. Also Mario Land 2 was the birth of Wario, this game's antagonist, which is welcome. I would've liked the series to continue introducing the occasional non-Bowser villain (e.g. Tatanga, Wart) here and there, but it's whatever.

I noticed Super Mario 3D World's reception has been on an upswing recently, but I'll still take it as the most underrated 3D title in the main series. IMO, Super Mario 3D World has the best OST in the series. Double Cherry Pass is my favorite tune in the series among other good pieces composed for the title. Level designs are polished and really solid and convinced me linear 3D Mario isn't a bad idea. I'm of the opinion both Crash Bandicoot and 3D Sonic games need to take a page from this game, because I've joked that this is "the best Crash game" because of its style. (I think Mount Mush Dash serves as one decent example.) Good visuals, great soundtrack, polished level design, and even if the multiplayer can feel messy, it can be chaotic fun. Hopefully it gets ported to Switch for those that didn't play it on Wii U.

I'm not sure what my favorite spin-off title is, but I'm hankering for a new Super Mario Strikers entry at the moment.