If literally tomorrow then I would afraid the damage that Ferengi could do considering current state of world economic and political trends. That is, if no religious group would call for a retaliation holy war against aliens.

I'd like to see Bajorans and Trill because these are most interesting species and pretty neutral and seem to be pretty realistic - neither possesses no special superpowers like reading on mind or superiority complex hidden deeply beneath everything you can see in person.

Vulcans are the obvious choice but the way they treated us after ww3

You gotta make up your mind if the universe we're living in is going to be ST one which would mean that everything we saw on screen over years was a historical representations of events to come which was passed in secrecy to Gene and other creators. Or, there's an parallel universe where the tv show events improbably happened and inhabitants are real and when this first contact (double first contact actually - as it happens to confirm both existence of extraterrestrial life and multiverse) happens we're having for example, our real-world Nana Visitor and an Bajoran individual Kira Nerys who looks like her who happens to be from the future, from parallel universe and so on with other people and characters. Either way, we're having a little advantage of some knowledge how to deal with these different species.