People usually treat GBA as a somekind of one trick pony that does only Pokemon or other JRPG games. I generally dislike the whole genre (apart from turn based strategies with rgp elements), but the GBA library is so much more, especially if you treat many games as small 5-10 dollar games and not the full cost titles they once were.
Some recommendations that do not come up every single time:
Racing Gears Advance
Ninja Five-O - if you manage to catch it cheap or play non-legit version
Chu Chu Rocket! - if my calculations are correct, there's something like 2500+ puzzle levels in this
Turbo Turtle Adventure - I actually liked this very much, reminds me of good DOS puzzle adventures. Youtube personels playing this suck, they skipped the tutorial/practice and then don't understand how the game works
Tony Hawk's Pro Skater 2-3
Medal of Honor Infiltrator - the top-down shooter, not the fps game
Scurge Hive
Wade Hixton's Counter Punch
I also like Metroid Zero Mission way better than Metroid Fusion. The years old debate.
If you have too much time on your hand, I actually recommend just going over to wikipedia's gba list, just scroll down the list and every time the name sounds even slightly interesting, just check it on youtube for a few seconds. I actually did this, but for me it was pretty quick as I dismissed almost all licensed rubbish, sports games and jrpg's without even looking at gameplay videos.