That's a tough one. I've played so many great games, it'd be tough to choose only one. but, if i were to try to, i'd say Ratchet and Clank: Going Commando. It had elements of RPG's, shooters, and platformers all rolled into one. What made it such an amazing game was the way it was rolled together. It very rarely felt like you had only one path to go, and it was full of secrets, interesting designs, and overall content. There was even a whole level in the game they tucked away, dedicated to showing you everything that went into the game, and some things that didn't. they even gave you things to play around with, like creating your own puzzles for certain gadgets, or playing around with laser blast effects. It had so much to do that even today it can compare to several big titles. It is without a doubt my favorite game from the franchise, and likely my favorite game of all time.