Measuring the euclidian distance from the normalized "quick count" to the voting preferences, the most accurate pollsters were: SDP Noticias-Covarrubias, Grupo Reforma, Ipsos-Bimsa, and UNO TV-María de las Hera. The worst performing pollsters were Milenio-GEA ISA and Indemerc.In some of the cases, the problem would seem to be motivated by a bias for Peña Nieto--Milenio's largely fawning coverage of Peña Nieto squared with its polling that vastly overstated his popularity. Although I also wonder if there's a frontrunner bias in Mexican polls--AMLO fell short of the polls giving him a wide margin in 2006, and, if I remember correctly, I believe there was a similar dynamic in the governor's races in 2010.
Update: The polls actually weren't so far off in 2006. Most leaned toward AMLO in the weeks leading up to the election, but nothing like what just happened this time around.