As Tom said, if you want to just play one, then Black/White or Black2/White2
If you plan on playing through the series
FireRed /LeafGreen -> Emerald -> Heart Gold/Soul Silver -> Platinum -> Black/White
Though you can skip emerald too and not miss much
--
http://img.imgcake.com/KommunistKoala/koalajpgehsa.jpg
http://img.imgcake.com/KKpngat.png