It's a little difficult to judge because most sales figures show the paired games as one total. If lots of people bought both games it's hard to compare that to an individual game. Most games beat the previous games for fastest-selling. From what I can tell, Platinum is the fastest selling game over a week, while Black & White hold the record for most pre-orders.
Anyway, here are all the total sales for the main Pokemon games (m=million). Red & Blue have the highest total, but they've had the longest to get that many sales!
Pokemon Red & Blue: 23.6m
Pokemon Yellow: 8.9m
Pokemon Gold & Silver: 23m
Pokemon Crystal: 3.8m
Pokemon Ruby and Sapphire: 13m
Pokemon FireRed and LeafGreen: 11.8m
Pokemon Emerald: 6.3m
Pokemon Diamond and Pearl: 17.4m
Pokemon Platinum: 7.1m
Pokemon HeartGold and SoulSilver 11.9m
Pokemon Black and White: 12.87m
