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?
I don't remember the name of it, but it was just a super troll hack where you're presented with the normal game for Mario 1, but then every obstacle had a hidden obstacle that would kill you when you attempted to play it as if it was the normal game.
There was another game made around the same time that was built entirely on that premise, but I'm pretty sure the Mario hack came first. I at least played it before I ever heard about I Wanna Be The Guy.
Other game: Syobon Action? ๐