Is ''Get out!'' an exclamatory or imperative?

It depends how it's used. If you are literally telling someone to leave, it's an imperative-- an order. But the phrase is sometimes used just to express surprise. "I saw alien in the street." "Get out!"  In that case, it's an exclamation, not an order. Many words can function in different ways. 

I would say both when written that way.  The ! makes it exclamatory.  the "get out" is already an imperative without regard to punctuation.