My favourite unofficial pokemon game would be pokemon insurgence.
The game have two versions of the story, a much darker tone, or a more traditional one tamer one. The dark story line isn’t child friendly, but the traditional one is completely fine. You can choose which one you want to play when you start the game.
While being an unofficial pokemon game, it has a really great story, great designs, and NEW MEGAS.
One of my favourite things are the alternate forms called Delta Pokemons, which are forms of existing pokemons with different typings, abilities, and appearances. The megas though are really great to see, some of the deltas also have some megas, but the already existing ones, most of them get a new mega form, like for flygon, the gen 2 starters, and even for the legendaries. Arceus and Giratina gets new forms as well. You can even get shadow mewtwo.
The game also lets you customize your appearance midgame. It also lets you choose your game mode, be it a normal run, a nuzlocke, or a randomizer, you can even combine the randomizer and the nuzlocke.
The game also has great quality of life improvements, the game has a special trainer that can help you train IV and EVs as well as the ability to speed up the game.
Long story short, it’s a very great game with amazing storyline, mechanincs, and tough battles. So it’s a really great game to pick up.