The Wii U didn't get much third-party support when it was quickly obvious that people weren't catching on. I could go on about it, but in keeping on the subject of Mario, Super Mario Bros. U isn't really much to write home about, and Super Luigi U is a remix but perhaps notable for being really hard if you play as Luigi (though ridiculously easy if you cheap out by playing as the invincible Nabbit). I did enjoy Super Mario 3D World, and I will never forget the 6.5 hours (and something like 600 lives) that me and my brother spent one Saturday attempting, and finally completing, the insanely difficult "Champion's Road", the true final area. Peach's floating and Rosalina's ability to attack are so crucial there... But if you say you haven't even finished the incredibly easy Super Mario 3D Land, then maybe you wouldn't appreciate the challenge of that stage in 3D World... However, it really is the "meatiest" Mario experience on the Wii U, though unless you have friends to appreciate the four-player simultaneous play in a 3D world (just a step up from the four-player simultaneous play in a 2D world in the "New" console series on Wii and Wii U), it may not feel like much of an evolution if you always expect bigger and better Mario experiences.
And as I mentioned, Super Mario Maker is great, but I never got around to making a stage, and the best stages by users often seem to be "automatic" ones, like Rube Goldberg machines, though indeed some are quite creative.
I regret to say that I never made time for Paper Mario: Color Splash, so I can't comment much on that, though I hear it's like Paper Mario: Sticker Star on the 3DS, which I also missed, in that neither are as RPGish as the previous games.
And the Mario sports games are meh, the newest Mario Tennis, whatever it was called, no one cared about, including myself, which is a shame as I generally at least enjoy them a bit. Of course, there's Mario Kart 8 which is fun, but if you missed out on the Wii U but are considering a Switch, might as well get the upgraded Mario Kart 8 Deluxe, then, which includes the DLC and adds new modes, including a "real" Battle Mode (there was a Battle Mode in 8, but used the same tracks instead of arenas specific to that mode).
Looking forward to Super Mario Odyssey for sure, which even Nintendo officially acknowledges is more in the vein of Super Mario 64 and Super Mario Sunshine (with its open hub worlds of Peach's Castle and Isle Delfino Resort respectively) than the somewhat fragmented Galaxy or "New" games.