Qix

As Ransom mentioned in his review, this 5200 cart pretty much nails the Arcade version of the game. Great sounds and really nice graphics make this a delight to play...and the 5200 controllers are actually fun to use with this game and work quite well.
Interestingly, this game is also available for the Atari 400/800 computers, but that version is much slower and missing a component of the 5200 port of the game, so this is the superior version of the home-version of the game. The 5200 has better sounds, graphics and speed.
Fun to play and a nice change from shoot-'em-ups and racing/baseball/soccer stuff on the Atari 5200.

This relatively obscure early-80s arcade game from Taito was one of my favorites. And this version just absolutely nails it! The two-button control adds immeasurably to the experience versus the one-button control on other platforms. And the graphics are just right. If you're a Qix fan, look no further. If you've yet to play Qix, you're in for a real treat when you pick up this game!