Formal:
* Vraiment (This is the most common and direct translation)
Informal:
* Vrai (This is more like "for sure" or "seriously")
* Ah bon? (This is more like "really?" or "are you serious?")
* Sérieusement? (This is more like "seriously?" or "are you kidding?")
* En vrai? (This is a more slangy way of saying "for real?" or "really?")
* Sans blague? (This is a more informal way of saying "seriously?")
Examples:
* Vraiment, j'ai gagné le concours. (For real, I won the competition.)
* Vrai, c'est incroyable! (For sure, it's incredible!)
* Ah bon? Tu vas déménager? (Really? You're moving?)
* Sérieusement, tu as fait ça? (Seriously, you did that?)
* En vrai, je suis trop fatigué. (For real, I'm too tired.)
* Sans blague, j'ai vu un fantôme! (Seriously, I saw a ghost!)
The best way to choose the right phrase is to consider the context of the conversation and the level of formality.