Which is better Reshiram or Zekrom

Reshiram is way better. You see, Zekrom is famous in America. Reshiram hit the roots in Japan, so it is more popular. Pokemon Black did sell more copies. In your battles between N and Ghetsis, Reshiram is more useful. Zekrom is weak to ice, when Reshiram is unique and is not. Reshiram can blast Kyurem, where Kyurem blasts Zekrom. Reshiram was created before Zekrom.
However, Zekrom gets all the credit. Reshiram's main type is dragon while Zekrom's is Electricity, so Reshiram's dragon attacks are stronger.