Pokémon

Unbounded was definitely one of the best.

A few others I like are Seaglass, Gaia, and crystal legacy.

I was recently blown away by Pokemon Emerald Seaglass. It features a complete graphical overhaul in a style I find absolutely stunning, an upgraded Pokedex, includes all Pokemon from Gens 1-3, adds overworld sprites for Pokemon followers, upgrades the battle system (things like inclusion of Fairy type, Phys/Spec split, z-moves), adds some new mini games, and way more.

Find out more on the official site.

Pokemon ROWE (Randomized Open World Emerald) is great if you like randomizers. All options are configurable in game.

Realistically, just check the featured hacks amd yearly award winning hacks on PokeCommunity. They're all great.

I'm a big fan of gen 2 so I mostly played romhacks based on that. To me the GBC offers a better experience than GBA because of the chiptunes compared to sound samples. Some personal recommendations:

• Pokémon Polished Crystal: it's pokémon crystal, but natures and abilities have been added, new movesets making lot of pokémon viable and interesting, and overall it's an enhanced experience from the original game. I fully recommend it
• Pokémon Prism: another romhack of crystal, but it's a complete overhaul of the game where action doesn't happen on Johto and Kanto, but on completely new regions Naljo and Rijon. It's an incredible experience, and the soundtrack is top notch (the gym leader music is amazing)

I enjoyed Pokémon Unbound and I'm working through Golden Glazed at the moment which isn't as polished and has some odd issues but is good fun.

“Pokémon pit of 100 trials” looks really fun. GBA romhack that plays like a rogue lite where you battle through 100 instances. Haven’t played it yet but it’s next on my list.