The main interrogative pronouns are: who, whom, which, and what. (Whoever, whomever, whichever, whatever, and whose can also be interrogative pronouns.)
Interrogative pronouns are used to ask questions. (The interrogative pronoun represents the thing that the question is about.)
Who won the race?
Which is the greater?
Which feat is the greater? (This is not an interrogative pronoun. It is an interrogative adjective — it modifies "feat".)
The versions with the suffix "-ever" are used for emphasis or to show surprise. They are quite rare.
Whoever would want to eat such a gross thing?
Whatever did you say?
Note that "whose" can also be a type of possessive pronoun called an interrogative pronoun.