I recently came across a colorization that turns the original black and white/green version of Pokémon Red for the GameBoy into a proper GameBoy Color title. This sent me down a bit of a rabbit hole, but the sheer number of hacks that have been made over the course of several decades is slightly overwhelming, so I'd love to get a decent first selection by hearing which are your favorites that have improved or transformed console and handheld games in meaningful or entertaining ways.
Thanks in advance!
Celeste Mario's Zap & Dash (NES): SMB1 turned into a Metroidvania with Celeste mechanics ported in. I think what impresses me the most is that they got 4-directional scrolling into this engine.
Super Metroid and A Link to the Past Crossover Randomizer (SNES): It's an absolutely incredible technical feat that this even works. SM and ALttP smashed together into a single ROM, with a few doors that take you from one game to the other, then the item pools are shuffled together so you have to go back and forth to find one game's items in the other. Unfortunately because ALttP is a much bigger game with a lot more items it kinda overshadows SM, you may not find this to be as replayable as the standalone randos. But I recommend trying it once because it's just so cool the first time.
Unfortunately I can't find an up-to-date download link for this one, just a few Youtube videos with no link, but there's an ongoing Panel de Pon GBC Restoration Project based on a lot of unused assets buried in the ROM before it tragically got reskinned (again, this poor IP can't catch a break). I've got an older build of this on my hard drive I could upload somewhere if anyone wants it, but the version I have is far from complete.