Which one is a versatile word, which can refer to an individual choice out of a set or a number of choices. It can appear on its own ("Which zebra will you choose?") or it can be combined with a noun that specifies the kind of thing in the set ("Which tools would you prefer to use?"). "Which ones" is another option, asking for multiple choices out of a group (e.g., "Which movies are for sale?").
It can also work as an object pronoun like he, she, and they (Give the document to whomever in the department). This article from Grammarly takes a look at how the word works in different contexts and some of its most common uses. Try Grammarly for free to make sure your writing is clear, concise, and correct. Which one do you think is the best spelling? Let us know in the comments!