I don't have much experience watching and zero experience playing SMW romhacks, but I think they're kinda cool. I quite like Grand Poo World 2, as well as the original Kaizo Mario World
Lunar Magic is also a cool tool, I've not seen a whole lot of how it works but based on what I've seen done with it and glimpses of the interface it seems pretty neat
Spoiler: quick thing about grand poo world 2