DS lite.
Best library (DS + GBA)
Best battery life
Best design

I'll probably go with the 3DS, as it is the one I have the most memories of gamewise. I didn't have a GB for long, the GBC was okay, as was the GBA. The DS came out at a bad time for me, around the time I got real busy with school. So I had less time on it than I did with the 3DS, and I've probably spent the most time on the 3DS. So that gets my vote.

Gameboy.

Pokemon Blue and red were amazing. Super Mario Land, Donkey Kong Land, Mega Man, list goes on.

I was never allowed a portable as a child, so I didn't grow up with them and was never really able to get into them.

I ended up giving my vote to GBA simply because it's basically a portable SNES and the SNES is my favourite console of all time.

i gotta go with GBA by a hair over DS IMHO GBA had the best library of them all, DS was good but it had weak Zelda games and way too many Cooking mama like games

You did well.

What's the point of the GBA when there's the DS? What's the point of the DS when there's 3DS?

3DS wins easily.

I grew up with a Gameboy, and I voted for the DS, because the poll didn't ask for the most innovative or impressive one--it asked for the best one. For me, being innovative or impressive are noteworthy, but what makes a system good (again, for me) is it's library and how enjoyable it is to play on. In these regards, I think the DS beats out the Gameboy pretty easily. (I absolutely love the Gameboy, but boy, having to sit at just the right angle to get light and avoid glare gets old fast).

The 3DS makes a good case for itself by being able to play the DS library and being slightly bigger (and therefore more comfortable on the hands), but it loses the battery life and GBA library of the DS Lite. And while the GBA SP has a backlight and can play GB and GBC games, I never found it's own library to be that compelling (I'd rather just play my SNES games at home)

The GBA can play GB and GBC games, while the DS can't.

The DS (original and Lite models) can play GBA games, while the 3DS can't.

This is a tough one. Eventually I went with DS, it has such a huge and varied library, and the DS Lite model in particular was excellent. It looked great, was fairly comfortable to hold, had a superb screen (easily the best screen Nintendo had done at the time), and had access to the GBA's excellent library to boot, which not only included fantastic original games but also had ports of some of the greatest SNES (and therefore greatest ever) games.

3DS would come second for me, because it has access to that enormous DS library, a good number of first class titles built for the system, and some great variety through Virtual Console and eShop.