Should "guess what" be a question or command?

Solution 1:

If "Guess what" is to be a complete sentence, it's unequivocally an imperative one. Logically, if not linguistically, the reply has to be a question itself…

Guess what.

What?

…unless the responder actually plans to guess.

Solution 2:

I think it can vary by speaker. My inclination would be to use "guess what?" when the speaker pauses to accept guesses, and "guess what!" when the speaker keeps on speaking, thus:

"I got something for you," Sue said brightly. "Guess what?"

"A book? ... a donut? ... a chicken?" Fred guessed.

"No, silly! The keys you left at the restaurant!"

vs.

"Guess what!" Sally shouted as she ran into the room. "I made bail!"

Solution 3:

I believe it should actually be

Guess "What?"!

Solution 4:

I think it is a command rather than question. The speaker is forcing / telling the listener to guess correctly about the fact in their context of speach. Because he knows the fact and now he is putting a puzzle in excitement in front of listener to guess it right.. Guess what!