Currently, it's Messi that's the best player. No doubt. His impact on the game is better than anyone else.. nearly every match he plays, it's Messi that causes a goal. There is no other player that can top Messi indefinitely right now.
His performance while playing for Argentina isn't that great because in Barcelona, the team revolves around Messi as the main goalscorer with Xavi and Iniesta as the support. However in the Argentinian squad there are numerous world class strikers and goalscorers in that team such as Higuain, Aguero, Di Maria, etc. You can't just come to the conclusion that Messi does not perform well in Argentina, he does. Players should be judged based upon their club performance because it's there they play most of their football.