For a long time my favorite was Excite Mario Bros.
Excitebike riders, Kirby, and Samus are trying to stop you at every turn from reaching the end of the levels! Here is a full game hack which changes all the levels and graphics. Also the music has been slightly altered to give the original music some change.
Recently though my favorite has become Celeste Mario's Zap & Dash!.
A hastily done minihack (with levels designed within 2 months) that features Celeste-styled gameplay.
It adds dash and wall jump mechanics. I haven't beaten it yet but I am really enjoying the level design.
What about you? Any favorites or recommendations?
Mario Crossover was also hours and hours of fun, but I’m not sure if it’s playable anymore. https://en.m.wikipedia.org/wiki/Super_Mario_Bros._Crossover
Play classic NES Mario levels as a bunch of other characters. Really great!
https://archive.org/details/SuperMarioCrossoverOffline