The Kanamit from the Twilight Zone episode "To Serve Man".
I had enough trouble with the Ferengi and Borg showing up 200 years earlier than we were previously told. So there aren't any races from the 24th century I would have wanted to see.
Maybe those little gold guys seen in the reception room during TOS: Journey to Babel. But I wouldn't want to see any TOS aliens before their established first contact.