I'd have to go with the DSL. I think that is when the DS just took off and never looked back, all the way to 154 million systems. That's what my head tells me.
Now, my heart tells me, NEW 3DS, I loved playing xenoblade on it, and the eye tracking 3D was amazing for me. I always have the 3D slider at maximum, so the eye tracking was really perfect me.
Now, as far as the most important, I think that goes to the GBA SP... it really fealt like Nintendo finally found some brain cells to use and brought them closer to the then current tech.