Houston should be ranked before Dallas. It's a much more International city which is the energy capital of the USA, has the largest medical center in the USA, and is home to NASA.
Dallas today is just another city in the DFW Metroplex. With Arlington getting the Cowboys, that area is going to start becoming the center of the metro area, while Dallas continues to decline.
1 Exxon Mobil Irving
2 ConocoPhillips Houston
3 Valero Energy San Antonio
4 Marathon Oil Houston
5 Dell Round Rock
DFW influence will continue to rise due to gas drilling/extraction on the Barnett Shale