To clarify what I believe SoulflameNinetales means, we sometimes think of games in terms of generations, like RBY (Red, Blue, and Yellow), GSC (Gold, Silver, and Crystal), RSEFrLg (Ruby, Sapphire, Emerald, FireRed, and LeafGreen), and DP (Diamond and Pearl).

Are you asking what the best generation is? Or are you asking about the best specific game? (And if it's the latter, should we consider all the games from RBY to DP, or only one or two generations?)