I think that the Gameboy line is definitely worth a nod too. All competitors have fallen by the wayside (Game Gear, Lynx, etc). We'll see what happens with the PSP but if history is any indicator, technical superiority and a higher price point is no way to beat Nintendo's handhelds. I have heard that Nintendo plans to keep the Gameboy and DS both in production but considering the rapid increase in popularity of the DS and the fact that it is backwards compatible with GBA games, I am wondering how much longer it will be until nintendo goes all DS on their handheld front. If only it were backwards compatible with the earliest Gameboy games and multiplayer capable with GBA games...
I just think it's awesome that Nintendo is still manufacturing a system that will play carts from 1989. I challenge anyone to find better backwards compatibility in the history of computing or video games.
I am surprised to see a couple of votes for Dreamcast as best system. My impression was that that system was esentially a flop.