My favourite Handheld?
I love all of them, but 3DS is the best at the moment.
Main reason is, it's a handheld where they will focus on hardcore gamers again, and the DS Market has been killed by piracy.
They were good in their times.
I still remember the revolutionary Original Game Boy, with a green/black display, best handheld graphics when it was available.
Either GBA or 3DS. The 3DS eShop may at some point have a whole bunch of GBA and normal Game Boy games, so I could then decide that the 3DS is better because it offers what the GBA has.
If the 3DS were backwards compatible with GBA cartridges, it would definitely be my favorite right now.
Gameboy Color, Gameboy Advance, and the DSi. All fun and enjoyable. They all also hold a special memory for me. Not all of them are brand new and amazing to most, but they are still special in my opinion.