What is your response to "sup"?

I do not associate with people who are only able to communicate using one syllable. 

i think they couldve said worse stuff

It's not often that this 70+ Scotswoman is addressed with "'sup?" but when she is, she says, "Oh hi, how're you doing?"

I usually say "Hey"

I have never been addressed by 'sup'.   But I am an older Briton.  Harking back a decade or three, my interpretation of "What's up?" is "What is wrong, what is the matter?"  It is a greeting to a group of people standing around with worried looks on their faces.

I say "t'sup" back.

Or the rare occational throwback to Wassssssssssssssuuuuup! (Wassup B?)

I say "No thank you."

This on the assumption that "Sup?" is an invitation to supper, in the same way that "Lunch?" is an invitation to lunch, that "Coffee?" is an invitation for coffee. 

I'm still wondering how to respond to a friend who asks "Jeet?" 

(Did you eat?)

nothin' dawgggg!

      I just say Hey