In my opinion the best games with value for your dollar for the Nintendo gameboy SP are the games that Nintendo makes. I would recommend both Warioware games and the Advance Wars games. Ones not made by Nintendo are the Castlevania games.
I really enjoy playing mario kart, donkey kong, mario brothers 3, ghostbusters, ssx 3. Don't waste your time or money on driver for sp because it looks like a stick figure running about, lol. I could go on and on!
The best games for the Nintendo GBA SP are any one of the Pokemon games. They have tons of content and very fun game play. They are also addictive, once you start exploring the world and collecting Pokemon of all different sorts in search of a perfect party on your way to the Pokemon League you don't want to stop!
I would recomend Mario Kart: Super Circuit,for a classic Kart-racing game. If you like fighters & Beat 'em ups then definately get Super Street-Fighter II & Final Fight.
Finally, Wario-Ware is brilliant for short gaming sessions with it's fast paced & funny mini-games.
Hope you enjoy my Recommendations!