I've always been a huge fan of Gameboy Advance. Its library was overshadowed by far too many Super NES ports, and some lazy design decisions by Nintendo (no X-Y buttons, no backlight, no dedicated sound chip) held it back for too long. The GBA SP model was a superb improvement over the original and the definitive version for me.
GBA was my platform of choice in those post-Dreamcast years, as I wasn't very much interested in the other 6th Generation consoles. I was probably a "retro" gamer by that point, spending most of my videogame time with classic system emulators. I would much rather be playing M.U.L.E. or Seven Cities of Gold on Atari 800.
What impressed me most about this system was its impressive collection of deep cuts, where all the best games lie. Hardly anybody had ever heard of Kelly Slater's Pro Surfer, Moto GP, Motoracer Advance, Gekido Advance, Virtual Kasparov or Hikaru no Go 2, all of which were (and are) spectacular. GBA is very similar to Sega Saturn in that sense, which is probably why I enjoy both so much.
Here are a sampling of screenshots for reviews that I wrote and published on my old arts website from long ago. The sizes are a bit small, due to smaller monitor resolutions and the need to optimize everything for websites (many people were still using dialup in those days). The screenshots were taken from the excellent Virtual Boy Advance emulator, which allowed me to dive deep into the software library. I also owned a GBA SP and a large stack of games.
Of the mainline franchise games, the Castlevania titles are very good, each better than the last, but always far too easy for their own good. Zelda Minish Cap is terrific fun and wonderfully designed. Lunar translates to the small screen very well, as does Tactics Ogre. Pac-Man Collection is always a must, although I really wish Ms, Baby and Jr Pac-Man were included. And Advance Wars 2 is the best thing to ever hit the Gameboy Advance. I only wish there were more save states for user-created maps.
Overall, an excellent little handheld that offers classic arcade thrills and a vast library with something for everyone. I'm grateful that Nintendo stubbornly held onto 2D videogames at a time when the classic style was all but extinct.