1. Donkey Kong Country 2, I had it on my GBA SP and played it from a very early age. Unfortunately though I lost the cartridge, so whenever I want to play it I just emulate it.
2. I guess this would have to be Legend of Zelda: The Minish Cap. It was the first Zelda game I ever completed.
3. Is this even a question? Stickerbrush Symphony of course. Also Forest Interlude, I like that a lot.
4. The GBA SP was the first game console I ever owned, and DKC2 was the first game I ever owned. It might have even been the first game I ever played.