I feel like ROM Hacks have a reputation for being harder, less polished, and more frustrating than your typical professional video game. I can't say that isn't an entirely unearned stereotype—they're usually made as a hobby in someone's spare time, so the extra polish of a real video game is usually missing.

Generally, I've only looked deeper into Super Mario World hacks, and out of all the ones I've played, I personally think the SMW hack JUMP½ has been my favorite hack so far.

Link to JUMP½ patch
It's not exactly an easy game, but it's far from ultra hard difficulty. The overall amount of creativity, polish, and crazy cool set pieces make it a wonderful experience. You can even unlock a shop that lets you buy a few items that make the experience a bit easier, such as an item that enhances your fireball attack, or an item that can revive Mario once per-life if he dies. It's not easy to sell it blindly like this, but I think if you're a somewhat skilled SMW player, you'll have a good time with it. It's been a while since I've played it and 100%ed the whole thing, but I have fond memories of it. Ah, it was only last year lol

Any other ROM hacks out there that are worth checking out? I know that Sonic the Hedgehog has an impressive hacking scene, so maybe I'll look there next.

Rom hacks were my life and soul back in the day, being that I didn't have a very good computer. For the Sonic scene I would recommend Sonic Classic Heros, Metal Sonic Hyperdrive, and Sonic Megamix.

Rom hacks were always fun to mess around with back in the day, but I feel like now it's a lot easier now a days to find really good rom hacks for example I started playing a rom hack of mario 64 that's called Super Mario 64 land I had a great time playing it even if I've been lazy and only done a few levels so far. xD

Rom hacks can be really fun, usually depending on what they do or how good they are. Like I went through a stage where I was basically addicted to pokemon rom hacks, they were a part of my every day routine at one point. I kinda miss those days but it just leaves me more time to be able to do other things I enjoy now too so silver lining? XD

I am constantly on the hunt for quality rom hacks. I shall name a few and shamelessly plug a few of my own romhacks as well. These can be found with a quick google search but feel free to message me if you are having trouble finding any particular hack.

Some are complete hacks which change everything some are just small improvements which make the game that much better.

N64:
Smash Remix
Super Mario 64 Land

SNES:
Super Metroid Arcade
Iron Man - Mushroom Avenger (One of my hacks!)
Mortal Kombat - Champion Edition
Addams Family Values - Afterlife Edition (One of my hacks!)
Super Mario World - Just Keef
Grump's Dream Course
Conker's High Rule Tail
Donkey Kong Country - Boss Blitz
NBA Jam - Old School Edition
Touhou Mario
Mega Man X3 - Zero Project
Oh No! More Zombies Ate My Neighbors
F-Zero - Eternal Boost
Final Fantasy IV Ultima
Mario Is Missing Done Right!
Mario Kart R
Hyper Street Kart

Sega Genesis:
Sonic Classic Heroes
Streets of Rage 2 - Syndicate Wars
Bare Knuckle 3 Project
Ultimate Mortal Kombat Trilogy (Works with only a few emulators, like RetroArch)
Sonic 3 Complete
Sonic 3D Blast - Directors Cut
Mortal Kombat II Unlimited
Megaman X in Sonic 2
The S Factor

Congo! : There are two particular hacks that I can think of that make certain SNES games more polished.

Super Ghouls 'n Ghosts - Super Arthur (https://www.vizzed.com/playonlinegames/game.php?id=99027):
Super Ghouls 'n Ghosts looks and sounds great, plays well with good level design, but the game is just way too hard for its own good. So this hack mitigates much of the unnecessary difficulty of the game to make it fair rather than cheap. For example, armor upgrades are also additional hit points (like how it should've been in the original game), you can swap weapons with the Select button which is really helpful, and the timer is a bit slower so you're not in any rush. It's really good, and actually makes Super Ghouls 'n Ghosts fun rather than a difficult slog.

Mega Man 7 Refit (https://www.vizzed.com/playonlinegames/game.php?id=102656):
Hardly different from the original Mega Man 7, but this hack makes a few minor touches to make it more polished. The most notable change is to the Charge Buster, as in the original game there was a small yet notable delay between when you hold down the button and when Mega Man actually starts charging. This hack has it so that Mega Man charges right away just like in Mega Man 4, 5, and 6 on NES. The font is thicker and looks worlds better, though typos still aren't fixed, and Mega Man has a bit more hitstun invincibility than before. This is the definitive way to play Mega Man 7!