Well I actually play gba on my psp these days so it's not actually "on my GBA" (I do still have a box full of them and games though, this one included) but right now I'm playing Zoids Legacy for like the 5th time through. No I do not watch the show or collect the action toys of the same name it's just a really fun game and I love the customization you have with all the parts and various other things. Anyone who likes rpgs and enjoying one would like playing this at least once.
It's between that and Gadget Racers right now, another game I bought wayyyyy back when sitting a shelf and taking a chance, I'm so glad I did too because I've played through it at least 5-6 times over the years. (This one I actually do play on a GBA, it's one of the few (if the only) games I've found that won't play properly on psp OR my Touch Pro 2 running Morphgear so I use a trusty sp or micro or ds whatever I grab first, and pop the cart in for endless hours of fun lol)