I usually avoid unofficial fan-made spin-off titles, but my friend had Newer Super Mario Bros. on his Wii and we ended up 100%ing the game over quite a few sessions. The game doesn't require any hardware mods, which is a big plus. I haven't played many co-op games all the way through in general, and 4-player co-op has been a feature of the NSMB series since the second game (New Super Mario Bros. on Wii, which this game is based on). So that was fun in itself, but the game has become well known for a reason: it's really well done and has some great surprises.
Newer Super Mario Bros. came out before the fun Super Mario Maker on Wii U, but the concept is pretty much the same: making a bunch of new levels from familiar existing Mario elements and game mechanics. Unlike Mario Maker, though, Newer Super Mario Bros. also had a team of talented programmers and artists who created new elements and locales not found in the original Wii game. Some are from past games, such as the Hammer Bros. suit and the Angry Sun from Super Mario Bros. 3, and some are entirely original. A lot of the music is also taken from other Mario games, including spin-offs such as Mario Party.
As is more often the case than it should be, this fanmade game has more creativity than many of the Newer Super Mario Bros. games. Whereas that series has really stagnated after the Wii game and each iteration since seems almost identical to the previous one, there are plenty of fresh ideas in this game, including brand-new locales and level hazards, new boss fights, and fun variations on familiar enemies such as giant Goombas that cause the earth to shake whenever they take a step. The game also has several quality of life improvements, such as the ability to save at any time and a different color on the world map to indicate stages that have multiple exits.
The game is quite polished, and there were only a handful of times that a stage didn't feel quite like what a legit SMB stage would. We didn't encounter any bugs whatsoever, and although the brand-new assets don't match the visual quality of the originals, that doesn't detract from the overall experience. As fun as this game is, however, it did make me want to go back and revisit the original games, which I've been meaning to do for a while. I probably won't get to it any time soon, but one can hope. ;)