Uranium is my favourite, mainly because it adds evolutions to some cool but forgotten pokemons such as primeape, dunsparce and corsola. It even has a fire/poison chimera!!
Being a nitpicker, I had always hated the official games for leaving behind some pokemon in a seemingly 'unfinished' state, so I particularly enjoyed playing this one even though the starters only evolve once.
Really, any of the games which add fakemons interest me, but recently I tried solar light/lunar dance and they were a big letdown. 300+ fakemons and almost none of them really stood out, I was not impressed.
So yeah, adding new pokemon to a game can make it that much more amazing, but not if they aren't impeccably designed and balanced stat-wise!
Screw game freak's quality, you must strive higher! {XD}
Another game that doesn't have fakemon and still managed to keep me interested is survival island. You must face the consequences of losing a wild battle, and you must literally try to survive at the start with only your starter ( which by the way, can be any non-evolved and non-legendary ). It emphasizes on exploration, something that clearly lacks in the official games. It even has ruins with puzzles!