I'd have to say the 3ds. Not only does it have the DS's excellent games, it also has a huge library and 3d that is actually now quite pleasant to use.

My second favorite handheld would probably be the PSP:go simply because the PSP was a beast as well.

tough one i like the 3ds and vita.

one has grate games one is a beat of a machine.

3DS and Vita. Closely followed by the GBA and DS... and PSP... Who am I kidding, I love all handhelds

without a question the 3DS

GBA!!!

Because of the eShop I choose the 3ds.
The Vita is a very solid 2nd place for the same reason.
The PS store has such a rich selection it's hard to not love the system.
Either way, great fun will be had.

Either the GBA or PSP.

In its prime? The PSP.

The original Gameboy was a beast but the PSP was everything to me! I had comics, movies, porn, music, games, pictures--it was vital equipment for me. I used to take it on dates with me, connect it to my car via aux cable, and have custom playlists! The Vita did everything better but, by then, it was obsolete. 3DS had a better library but lacked the "cool factor".

PSP. Final answer.

... And they say i have an anti-Sony bias.

PSP, I love the gameboy advanced but the PSP did so much more (obviously more advanced) but it was basically my first all-in-one machine.. I had a DS but i didn't get much out of it.

I don't have a favorite handheld. I like all of the ones I've owned (GBC, GBA, DS, 3DS, and PSVita).