kpp mentioned yoshi vs windows, so i guess that. it's certainly representative of what fangames were like in the earliest days of mfgg, and that's its greatest strength. also, supertoad2k never shut up about how much he hated windows, so it's no surprise to me that his magnum opus would be something like that.
if not that, then the old fangames that thunder dragon did like marioquest and wario's revenge, even though they're not very good by today's standards. those set the stage for a lot more games like them, and they're nostalgic to play besides.