Black / White for the very good story, stunning presentation and overall great new Pokemon. Followed very closely by HG/SS for the great combination of nostalgia with new comfort, epic quest over 16 gyms, an insanely tough last battle and the cute feature of seeing you with your Pokemon walking around.
I recently played Yellow on a Super Game Boy and... man. How "lame" the new editions might be to some, actually knowing the exact stats of an attack or having as many items as you want isnt something to overlook. They are still groundbreaking for their systems, but I realy got used for the more transparent and comfortable new games.