More literal translations:
* Tu me fais marcher? - This is the most direct translation and implies you are being tricked or made fun of.
* Tu rigoles? - This is a more casual way of saying "You're kidding, right?"
* Tu déconnes? - This is a slangy and informal way of saying "You're messing with me?"
More expressive options:
* Tu te moques de moi? - This translates to "Are you making fun of me?" and implies disbelief and perhaps annoyance.
* Sérieusement? - This means "Seriously?" and can be used to express shock or disbelief.
* Tu es sérieux? - This translates to "Are you serious?" and carries a similar meaning to "Sérieusement?"
Informal options:
* Non, mais sérieux? - This literally means "No, but seriously?" and is a playful way of expressing disbelief.
* Pas possible! - This means "It's impossible!" and conveys a sense of shock.
The best way to say it depends on the situation and your relationship with the person you are speaking to. Choose the phrase that best fits the tone and context of your conversation.