That is a tough question. Not to sound like a fanboy, but I would say PSP. (pretty much a PS2 in your pocket with a great collection of games.)
Others I love would be the Sega Game Gear, the Turbo Express (a portable Turbo Grafix 16!), and the Gameboy Advance SP (more compact than the first GB advance model and it introduced the internal front light.)
I still own my original launch Gameboy and Gameboy Advance. Never bought the "Gameboy Color," nor have I bought the newest Nintendo handhelds.
But, I do want to buy a PSV eventually.
Ohh from what i remember i had soooo much fun with the Gameboy Color!!!!
I was bought the gameboy color and Pokemon Crystal. i spent HOURS playing that game i took it everywhere with me and just kept playing it.
Please somebody make a time machine PLEASE
I had a PSP but i definatly did not have as much fun as i did with the gameboy color. i miss those gameboy color days
Planning on getting a PS vita though and i do like my ipod touch (4g)
someone make a time machine please and let me use it
The most useful and current favorite? The Vita. Great browser, great cameras for Skype calling, a fantastic screen, multi-purpose and I played many more games on it than the PSP.