I'm going to play through 255 of the best games for the GBA. That's 4.9 years at a rate of 1 game/week. I'm not sure whether that's a good estimate yet.
To get the list, I took the games with >=70 score on Metacritic, filtered out some games in genres I don't enjoy (mainly sports simulators and serious racing games). That conveniently came out to 255, and since that's a significant number in gaming, I decided to make that the title/theme of the challenge. Here's the finished list.
This self-challenge was inspired by internet sites like CRPG Addict, who is playing every CRPG ever made in chronological order, and NESMania, where speedrunner "TheMexicanRunner" beat every NES game ever made. I wanted to do something similar for GBA, but (according to Wikipedia) there are over 1000 GBA games, which made playing them all a bit much. So instead I came up with my arbitrary challenge list via the method above.