Here's my top 3, in no particular order....
1. L.A. - Too large of a city not to have a team.
2. S.A. / Austin - Two big cities, and Texans love football, duh.
3. Vegas - City's growing fast and nobody does things like Vegas does.
Dark horses:
Fort Worth - I'm kinda suprised nobody mentioned Fort Worth. It's the 20th largest city in the US and growing fast. Put them in the AFC, that way they wont have to compete with the Cowboys as much for fans. We could have our favorite AFC team to the west, and the best team ever to the East.
Rapid City - Not a big city, but people would come from all over the state, and surrounding states to fill the stadium, afterall, there's nothing better to do. A city set in the mountains, this franchise would be a alot like the Packers. Lincoln Nebraska was mentioned before, I like that idea too, but think it would be better in Rapid City, the CornHuskers will always be top dog in Lincoln. People in this part of the country are pretty big football fans, but they're either Bronco fans, or Viking fans, cause those are the closest teams to them.