Which of the following is correct?
- Which one do you want?
- What one do you want?
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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.