You're not serious, are you? (冗談でしょう?)
以下、全て同じ意味。冗談でしょ?
You gotta be kidding me.
You gotta be kidding.
You've got to be kidding.
You must be kidding!
You must [have got to] be kidding!
You've got to be kidding!
You can't be serious.
You're joking.
You've got to be joking.
You're joking.
You're kidding.
You must be joking.【by英辞郎】