GBC:

• Game & Watch Gallery 2: Holds a special place in my heart as the first game I ever owned. Has the best lineup out of all the collections, with 3 and 4 you can kinda tell they had used up all the heavy hitters.
• Mario Tennis: An incredible tennis RPG. And Mario doesn't even show up until the postgame as a bonus boss, which I find hilarious. Has connectivity with the N64 version if you can get that running, lets you transfer your RPG mode character and unlock more content on both titles.
• Panel de Pon GBC: Better known under a name of a different IP it got reskinned with, but I'm a stubborn snob who will only ever call it by the original title. It's a bit different from the console versions in order to compensate for the small screen, board is shrunk from 6x12 to 6x10, and the 1P Arcade mode is fake versus that gives opponents a health bar rather than their own board. I actually have a soft spot for this version, it's different enough to stand out and be worth enjoying on its own, even if Gamecube is still the GOAT.

GBA:

• Boktai trilogy: Hideo Kojima's greatest masterpiece. First game's alright, second game is where it comes into its own. Note that you want the Solar Sensor hardware for the full experience, but emulating them is worth it over not playing them at all. And for the third game, you'd have to pick between original hardware or the translation patch anyway.
• Castlevania: Aria of Sorrow: It's Castlevania. It's good. Also check out Circle of the Moon and Harmony of Dissonance, but AoS was by far the best of the GBA entries.
• Golden Sun 1/2: These games were way ahead of their time for how they designed a combat system that encourages you to use all of your tools and not just click basic Attack as if you gotta hoard your MP for a rainy day. Fantastic puzzles too.
• Mario & Luigi: Superstar Saga: If you've played any of the other Mario RPGs, this one's great too. Has a 3DS remake but I haven't played that version so I can't tell you how it compares.
• Metroid: Zero Mission: The original Metroid has aged rather poorly if you ask me, but this remake does a perfect job modernizing it into one of the best games in the series. Fusion is good too, but some fans have opinions on that one.
• Mother 3: Surely you have already heard of this game and do not need me to tell you to go play it. Have you not played it by now? Why not? Well, okay, if you haven't played Earthbound first, go do so, then play this.
• Rhythm Tengoku: A wonderful game about pressing the A button. Sometimes you press the d-pad too. Translation patch.
• Summon Night: Swordcraft Story 1/2: If you've ever played the classic 2D Tales games, these are excellent spiritual successors to those. There's a third game that's JP-only, translation patch is being worked on but it's been stuck in development hell for years...