There are lots of ruins in Mexico, so you need to give us more info.
In the Yucatan.
Chichen Itza is the most impressive
Uxmal is close in impressiveness, and in my opinion is the most beautiful.
I would rank either of the above as a "most see" if you haven't seen Mayan ruins.
Tulum has terrific location, but the ruins are much less impressive than those at CI or Uxmal.
San Gervaso - I wouldn't bother going there unless you've never seen ruins and these are the only ones that fit into your schedule.