Hands down, Pokémon Mystery Dungeon: Explorers of Sky! The game has one of the most amazing stories I've ever seen in gaming! Not only that, but there are SO MANY things you can do after beating the game it's unbelievable! The levels are randomly generated so there will never be any monotony and, get this, there is an UNLIMITED number of side quests you can do! Not only that, but there are story segments called Special Episodes that focus on characters other than the main character. Seriously, this game is packed with so much content it's unbelievable! I've recently started experimenting with hacking my save file so I can play as any Pokémon I want. You can get in on the Nintendo DS or Wii U Virtual Console, but you can't hack the Wii U Virtual Console version.
But everyone just hates on this game and puts it down simply because it's not the main series. I HATE it when people say that they don't like my favorite game for such a STUPID reason! Have they even played it and experienced it for themselves!? This game is my favorite game and is the best game I've ever played! It got released in 2009, I got it then, and I'm still playing it today! And, in that game, you play as an actual Pokémon instead of a human training a Pokémon and I like playing as a Pokémon much better!