It depends. If you want to start with the originals, then G/S/C is best, because it has Kanto and Johto. Or get R/B/Y. If you want newer than wait for HG/SS, but don't get D/P/P. They're terrible compared to other games in the series.
Ok, here's the real deal, play all of the 'third' games to get the best of all worlds and get the gist of pokemon. First play yellow, then crystal, emerald, and platinum. Then choose any game you want to play from there on out.