I think Spock of the classic series goes down first, but I think this is a close fight.
I think even classic Khan and Spock make it a good fight...but I still give Slade's healing the edge, and comic book characters with experience fighting more powerful characters the edge.
In general, I think Spock and even classic Khan are peers in strength to Slade and Cap...but the're not as fast, agile, or as good at fighting.
Transplanted, however, I think Khan would end up being very much like Slade as a villain....
Spock though was shown and said to be a Vulcan master of martial arts, and Khan had a healing factor also, so could still win this!
Spock would be maybe 3-5 X peak human, Khan about 10X