As far as multiplayer action goes on the GBA, you can't go wrong with Mario Kart Super Circuit.
This is as good as it gets. Intense, competitive and fun multiplayer gaming on the GBA. Though, you'll each need a cartridge of your own to get the most out of the game.
Single-Cart action with multiple GBA's limits you to one character Yoshi(with different colours) and four tracks if memory serves me correct. But, even with a Single-Cart, the game can be highly enjoyable, competitive and fun. How many players you have participating in a Multiplayer game is directly proportional to the amount of enjoyment you'll get from it.
2 Players = "An Enjoyably Fun and Intensely Competitive experience"
3 Players = "Multiply the above by 5"
4 Players = "Multiply the above by 10"
Gameplay 10/10 It doesn't get any better than this(multiplayer)
Graphics 9/10 For a GBA game it don't get much better
Sound 8/10 Good Game Music/Sound Effects
Value 10/10 If you own a GBA, you need this game in your collection
Tilt 8/10 The single-player experience isn't much, but, the multi-player is unmatched on the GBA.
Mario Kart Super Circuit 9.1 superb