Here's a list of good/great GBA games in no particular order (that happens to be alphabetical):
Advance Wars 1 and 2 Castlevania games (3 of them) Crash Bandicoot for GBA (both games) Donkey Kong Country 1, 2, and 3 Doom for kicks Dragon Ball Z Super Sonic Warriors (probably more for DBZ fans though) Final Fantasy ports Final Fantasy Tactics Advance Fire Emblem 6, 7, and 8 (if you pick one I'd go with 7) Golden Sun games Harvest Moon Friends of Mineral Town Kirby Nightmare in Dreamland and Amazing Mirror (especially Amazing Mirror) Klonoa games Legend of Zelda - Minish cap and LttP (Four Swords is also pretty fun if you can get somebody to play with you) Mario & Luigi - SuperStar Saga Mario Golf and Mario Tennis (these are actually good ass games, top 20 GBA games imo) Mario vs. Donkey Kong Metal Slug Advance (has a lot of replay value with fun unlockables) Metroid Zero Mission and Fusion Monster Rancher Advance 2 (I haven't played the first, but I think 2 is basically the same but better. Also this game might kinda suck but I have nostalgia for it, so I think it's fun) Ninja Five-0 Pokemon R/S/E and FR/LG Pokemon Pinball - Ruby and Sapphire Sonic Advance seroes Summon Knight 1 and 2 Super Mario Advance series WarioWare games
There are tons of others that are notable that I haven't played, so if your favorite GBA game isn't there I probably haven't played it.
Super Robot Wars Original Generation 1 and 2 Megaman Battle Network series out side of MMBN1 Sonic Battle was pretty cool too
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I don't know why I'm playing Destiny 2... but I am...