Best game...That's so tough!
I have to give out a tie. Golden Eye for N64 or Splinter Cell: Chaos Theory for Xbox.
Those are my favorite games. I say they're the best because you can't really name an FPS without Golden Eye coming to mind, and the countless hours and sleepless nights spent split screening this with my buddies is priceless.
Chaos Theory...just about as many memories. From the late nights playing Gamebattles matches, to saying "I'll see you later" to a friend I met over live before he left for the Marines and later for Iraq. Not to mention the game play. It took a new genre, introduced by it's predecessor, Pandora Tomorrow, and improved on it's award winning design. Ground breaking game play that will never be rivaled.
My worst game is kind of ironic. It's a Splinter Cell title, and it's also the most recent. Double Agent was crap. Terrible, nigh unplayable crap. The single player, though supposedly innovative, was a let down. The multiplayer took a proven formula and threw it out the window in favor a noob friendly antics that gave me a few morsels, only to leave me wanting more to chew on like a emaciated Ethiopian.
My best games also had great campaigns. Chaos Theory scored a 9.9, the highest for any Xbox title. It took the given hardware and made an incredible story, with great graphics.
Golden Eye, like said before, just paved the way for console shooters.
Those are mine.