I agree with alot of the choices here, you should also look at the following as also rans
Juka and the Monophonic Menace. Nicely drawn and good look to it though a bit on the simplistic side gameplay wise
Karnaaj Rally, love the top down race look to it. Done by the people who did Skyforce on the PPC, real nice graphics for a GBA
Banjo Kazooie Grunties Revenge. I liked this and thought it looked good considering the limitations of the GBA. Kept within the spirit of the N64 Games
Final Fantasy Tactics Advance I thought this looked and played great
Monster Truck Madness, 3D graphic are amazing considering, go on try it and at 4mb tell me your not impressed
The Legend of Spyro: The Eternal Night Look at the release of the game on the DS and then this. The GBA wipes the floor with it in every way, Graphically great and very playable.
Medal of Honor - Infiltrator Very impressive sound and audio, nice speach, loved the game, the first Medal of Honor was dire
Yggdra Union: We'll Never Fight Alone Very impressive graphics
That said my favorite most outstanding games on the GBA were
Fire Emblem - The Sacred Stones
Legend of Zelda, The - The Minish Cap
Pokemon FireRed and Leaf Green
Mario and Luigi
Astro Boy
And the top of the list Kingdom Hearts - Chain of Memories just to hard to fault it on any level. 10/10 in my book