I would say the my favorite series of The Big Bang Theory series 5! mainly being because it includes monumental events such as the planning of Howard and Bernadette's wedding and also the fact that Sheldon officially enters into a relationship with Amy :)

But what about series 9, when Sheldon and Amy finally have Coitus? Because that was a long time coming, and they even go as far to create an episode of them trying to have sex for Amy;s birthday the next year.