Which of the following is correct?
- Which one do you want?
- What one do you want?
If you're offering a limited choice, then use which: "Which of these 25 models do you want (to buy/try/look at)?"
But if there are endless possibilities, use what: "Now that you've won the lottery, what do you want to buy, a Rolls, a Benz, a Ferrari, a Lamborghini?"
"What one do you want?" isn't idiomatic English, but "Which one do you want?" is.
In American English, which one is a common phrase. What one is not, and it sound very stilted.
Which is used to refer to a specific object, while what is referring to a more general selection.
In your case, "which one" do you want is correct. You can also change it to be "what do you want".