I would just wait for Heartgold and Soulsilver.

I have played all the major Pokemon releases. The game I had the most fun with and played the most was Emerald. But depending on if you want to play the most recent games or not, you have a lot of choices.

If you want to start from the 'beginning', you can play the remake of the first game by buying either Fire Red or Leaf Green. Or you can play Emerald (which is essentially the best version of Ruby Sapphire). Or you can buy Platinum (which is the best version of Diamond/Pearl). Or you can wait for Heart Gold/Soul Silver (which is a remake of Gold/Silver).

Platinum is the best pokemon game currently available. Get that.

Yeah, Platinum is the best one you can get at the moment, hands down.

Just wait for Heartgold and Soulsilver, you won't be dissapointed.

This. It has the most to do after you beat the game, and the ability to use all the older pokemon. Waiting for a Gold and Silver remake that won't be out till next year seems silly.