Tae Kwon Do for most of my life, and for the past 6 years MMA.
I wouldn't downplay or take anything away from a person with a Karate or Tae Kwon Do background. I think people who have a solid background in one of the stand up arts have a much better advantage coming in to the MMA game.
Look at Georges St. Pierre and Anderson Silva. Two of the best fighters in the UFC, and they came from Karate and Tae Kwon Do backgrounds. The days of the wrestlers (i.e Tito Ortiz and Matt Hughes) are over.