Easy..3DS, classic Nintendo handheld experience, decent graphics, decent online, stereo 3D, great E shop and more importantly good games.

Never been a huge fan of handheld gaming in general, but if I had to choice I'll go with the Gameboy Color, only because of Gen 1 Pokemon :p

GameBoy Color. It beats DS and GBA. Also, obviously the original GameBoy. I found the GBA to be mediocre. In fact the entire 6th generation is my least favorite. The DS and the 7th gen fixed that somewhat.

However, by the looks of it, the 3DS has a very good chance of ending up top. It's already beaten DS and GBA in my book. That bodes well for the 8th gen as I see good things for WiiU too!

Easily the DS for me, specifically the Lite model for its beautiful design and screen quality. The DS was a bona-fide breakthrough for gaming interface.

I loved the GBA as well (specifically the SP design) and the 3DS is also great, although it's still too early to express just how great.

3DS to be number one of all time..... Maybe!

My Rank:
1. GBA- Several of my favorite handheld games came from this, also liked its small design (had sp).
2. DS- I liked a good amount of games, but no favs really. Design was great.
3. GBC- Only had 2 games, but I loved them.
4. 3ds- While I only liked 2 3ds games like the GBC, I tried many games and was very underwhelmed by them. You know how it is, sometimes it's best to not experience something.

Game boy color by a mile never had another one after that.