Formal:
* Que regardez-vous? (This is the most formal and direct way to ask.)
Informal:
* Qu'est-ce que tu regardes? (This is a more casual way to ask.)
* Tu regardes quoi? (This is even more informal and common among friends.)
* C'est quoi, ça? (This literally means "What is that?" and can be used to ask what someone is looking at, especially if it's something specific.)
Adding a little bit of attitude:
* T'regardes quoi? (This is a very informal and somewhat rude way to ask.)
* Tu regardes ça? (This is a more casual way to say "Are you looking at that?")
The best way to say it depends on who you are speaking to and the situation.