I've played quite some good ROM hacks before, don't mind sharing them with you.
- As stated before, Snakewood, with a ROM base of Ruby. Tbh, good in the beginning, but gets a little too sloppy nearing the end. Although there are still funny plot twists, it lost its appeal to me.
- Fire Red Omega, which has also been said before. ROM base of Fire Red, of course. Basically what Chubb said it was. Definitely worth trying, although it's not nearly as hard as the other Drayano hacks.
- Dark Rising (and Dark Rising 2) with Fire Red ROM bases. Kind of ridiculous level curves, but an interesting storyline. Dark Rising has been finished but can be buggy at times, while I'm not sure about Dark Rising 2, as I haven't played it.
- I dunno anything about Gaia, but iirc, it hasn't been finished yet.
- Ash Gray, with a ROM base of Fire Red. This hack follows the journey of anime protagonist Ash Ketchum. Really, try this one. It's great in my opinion. It also gets updated quite frequently.
- Pokemon Adventure - Red Chapter, with a ROM base of Fire Red. Follows the story of manga protagonist Red, with a few tweaks of mons and such. A really great game I can recommend playing.
- Sweet Version, with a ROM base of Fire Red. Try. This. One. Custom type chart, with types like Blueberry, Raspberry, Strawberry, Cherry and Chocolate! Custom region, custom areas, PokéSweets instead of Pokémon... Seriously, what more do you want? Get it.
- Alternate Evolutions, with a ROM base of Fire Red. Legendary. It's basically still the same as regular Fire Red, but made more difficult. I'm talking more difficult than Fire Red Omega. The best thing is that the evolutions have been changed. Like, Squirtle evolves into Wartortle, but Wartortle doesn't evolve into Blastoise, but a different mon instead! Really worth trying.
- Ruby Destiny series, with a ROM base of Ruby. They're all pretty good, but I can especially recommend Life of Guardians. You can choose yourself whether you want to be on the good or the bad side at a time in the game, or if you'd rather go neutral. Custom mons are also in. Really great game.
- Flora Sky, with a ROM base of Emerald. Another game that was really well made. Never made it to the finish before, but one of the games I could recommend, seeing how far I've played. Also one of the only hacks that uses Emerald as a base.
I'm pretty sure I forgot some hacks. When I think of others, I might put them up here too.