As far as I know, the best selling system made by Nintendo so far is the original GameBoy (original + GB Color units), with about 120 million units that were sold (or am I wrong here?)
At the rate the DS is selling, it´ll soon become Nintendo´s best selling system ever....you all see the weekly charts and know what a monster the thing is...back in 2004 I´d never have imagined that 'ugly' system (original phat model) would go on to be the system that not only will soon become Nintendo´s best selling system, but to also be the first system to really have the potential to surpass the PS2.