These 3 popped into my head first as the greatest game I've ever played, so I narrowed it down to 3. Heavily recommend anyone try these 3 out if they enjoy games that involve a lot of creativity, and/or are looking for something slightly Pokemon-esque, but far more fleshed-out and far superior imho. All 3 of these games continue to blow my mind to this day, and I always go back to them on occasion.
1. Jade Cocoon: Story of the Tamamayu (PS1) - Take Pokemon, then make it far more adult-thematically, and now your captured monsters can be combined with other captured monsters to power them up. This opens up everything, as you will combine their stats, combine affinity types (fire+water types = a fire/water hybrid with unique hybrid moves), AND combine their visual appearance right down to 3D model and texture. Needless to say, your monsters truly are your own. This was mind-blowing then on PS1, and I have yet to see a game do this so well to this day. Then again, same can be said of all 3 of these games.
2. Magic Pengel: The Quest for Color (PS2)- A turn-based game (again a bit like Pokemon) but you literally draw your characters into existence, and their stats and abilities depend on how you draw them and which colors you use. It still amazes me how intuitively your characters animate exactly how you picture they would. You can make a straight line that will just bounce around and but done with it -- but then you can draw it with a circle at the top for a head and how it headbutts. You can then draw a face on the head, and the face reacts with expressions when it does attacks or gets hurt. You can then add a "jiggly brush" to its mouth and give the appearance of drool that jiggles there, and that drool gets incorporated into a drool fireball of sorts. The game just decides on its own how your drawing will be used, and could completely change every time you decide to change your character. Again, it's mind-blowing to this day.
3. Graffiti Kingdom (PS2) - A sequel to Magic Pengel with many similaties, but this time it's an action game. You can make a wheel that moves like a wheel, then give it arms that give it a specific uppercut attack, and then draw it with flames that give the uppercut fire properties and a fire-breathing attack. The fire may make the wheel roll faster. Since there are now platforming elements, you can strategize to switch between certain characters that move (and battle) in different ways -- of course this all depends on how you drew/built them.
Side note: If you do intend to play Jade Cocoon, AVOID its sequel Jade Cocoon 2. Ironically its the most disappointing game I've ever played, as a completely different dev team took it over and removed everything that made the first game great.