If you have a GBA and don't already own Mario Kart, then shame on you.
Addictive. Infuriating. Fantastic. Mario Kart is arguably the greatest race game of all time and this GBA version has all-new Grand Prix competitions as well as the original SNES tracks, which are unlockable when you get the elusive Gold Cups.
The graphics are stunning considering they're on a handheld, being somewhere between the SNES and N64 in quality.
4-player Battle Mode with game-link is the icing on the cake. You must own this game!