Xni-Pec in Cicero:
http://www.lthforum.com/bb/viewtopic.php?t=11753
Try the vapores (tamales), panuchos con consome, and a few side tacos of cochinita pibil or any of the weekly
rellenos while you're there.
Tell Antonio or whomever serves you that you know your fish, and they'll make sure to give you the best of the batch, or will decline outright to serve you if it's not at the peak of freshness (in which case, eat other things, and try back again). I find their snapper very authentic and delicious but also quite a mess to eat between the bones, banana leaves, and sauce, and it's not worth it unless it's fresh from the market / truck.