The classic GBA! It's like the sexiest handheld ever imo (together with the Vita), that form factor is just too good.
Only shame is the dark screen, but everyone and their mother mods them nowadays.
I had this one, which was my first gaming system.
I was the first one of my friends to have a GBA, they all had GBCs and I felt like the king of the schoolyard. Apparently the first thing I asked my parents when I got it for my birthday was how they could afford it, since we were really poor when I was little.