The GBA SP and the PSP Go are by far my favorite handheld designs of all time, and I really miss small devices like that.
The way they feel in the hand, the ease of pocketing them, the screen size, they're absolutely unmatched for the purpose they were designed: carry a gaming device everywhere.
Though hard to find and pricey, the psp go's dock makes it a great desk companion. I love having it display the clock, but its so pretty I cant help looking at it and immediatly pick it up to play