Super Mario World romhacks. These are the ones that stick in my mind now, a decade after my interest in them peaked:
Return to Dinosaur Land - pretty sure this was the first one I ever played, shortly after discovering the romhacking scene. Has the distinction of being easier than the original game when most romhacks are much harder. Romhacks are distributed as patches that contain only the differences between the original SMW rom and the hack, because it is illegal to distribute the full romhack. I applied the patch to a SMW rom that I had fucked around with, e.g. making the palettes stupid, instead of a clean SMW rom that you're supposed to do it on. I might go and play the hack again properly one day.
Demo World: The Legend Continues - made by the same guy who made the SMW level editor. Difficult and punishing and full of secrets behind secrets. I think it was the first hack with Super Mario Brothers 3 music. For many years, SMB3 was the only alternative music there was, until some Japanese guy came up with a tool called AddMusic. A lot of cool stuff came over from Japan actually, like Blocktool for inserting blocks and terrain that could have custom effects (eventually succeeded by Blocktool Super Deluxe that inserted more efficient assembly code into the rom), Spritetool for inserting custom enemies, Addmusic for adding music, and a bunch of cool romhacks too.
Brutal Mario demo - a romhack from Japan. Notable because the maker's custom sprites, custom bosses, and custom level-specific code in general were really advanced for the time. The level design left a lot to be desired though. Unfinished, and will never be finished because the guy has a wife and kid now.
VIP and Wall Mix series - five completed collaboration hacks from 2ch, lots of references to their memes, eclectic level design and custom assembly code especially as you go further into the series. There is a sixth WIP entry currently almost complete but stalled for the last few years.
Kaizo Mario World - Japanese hack, ushered in a whole genre of hacks. The idea is that the levels are so difficult and demanding of pixel-perfect precision and perfect foreknowledge that you can only feasibly complete them with emulator savestates. Speedruns of them look cool as hell.
Xka Shack - one of the founders of SMWCentral, the main SMW romhacking community, rickrolled everyone. For a few months he had been pretending that he was working on a hack using a tool called Xkas only intended for applying assembly code patches to a rom.
Super Mario Infinity I and II - Some hacks made by a guy who was really into the Paper Mario series and wrote fanfiction of it. These hacks had a whole bunch of plot told in rudimentary cutscenes. The levels were long and difficult and not too great (both hacks each had a notoriously long level that took 15 minutes to complete), and the graphics in many places were kind of shit or had bad palettes. I loved these hacks anyway.
The Second Reality Project series - Hacks made by a top tier level designer. They had some remakes which really polished the graphics and music nicely while also adding new endgame content. The third one in the series has been in development for over a decade and probably won't be finished. That's too bad, because there was a lot of hype in my mind for it.
Panic! in the Mushroom Kingdom I and II - Your standard mid 00s romhacks, with custom graphics and SMB3 music and middling level design, but they stand out because they both had secret worlds with a 'Palace of Hentai'. All the 256x224 pixel, 16-colour wallpapers of anime cunny you could ever ask for in a romhack.
Mario's Keytastrophe - An unfinished hack whose main gimmick was that each level was a puzzle about putting a key in a door. The guy making it was really into music and composing custom music and the hack slowly became more about music than the original concept.