Most socially conservative large city (pop. over 600,000)

[Sam Spade]

With the Texas cities, there is an important distinction between city and metro area. 

Both the cities of Houston and Dallas have sizable gay communities (Houston larger than Dallas) and are full of socially moderate Republicans.  Houston has a sizably greater Hispanic population than Dallas, both have similar black populations.  Both these populations tend to be socially conservative on most issues.  Houston has a good bit more conservative suburbanite Republicans with city limits than Dallas; this has a lot to do with housing concerns.  Therefore, I would clearly say that Houston is more socially conservative than Dallas by a good bit.

When you get into metro areas, that's where the conservative Republicans are in total control in both places.  In metro areas, there is little difference between Houston and Dallas, except that I would give the socially conservative edge to Houston because of the Hispanic thingy.

[Sam Spade]

Cities that have annexed what would normally be suburbs in other cities are generally more conservative and Republican. Houston is the best example and is why the city has over two million people. However, I would guess that Jacksonville is the most conservative.

Houston is hard to compare to other cities, since it's really one big suburb, and the city has a severe hatred towards urban planning.  This means that if a suburb is closer to the center of the city, it is not necessarily more conservative and Republican.  Witness northeast Fort Bend County vs. southwest Harris County, where this trait can be found all over the place.

FYI, I should note that the city of Houston is about 61-65% Democrat.  The other 1.6 million in Harris County votes about 70% Republican, giving the total area its generic Republican lean of between 51-55% Republican.  Of course, a good number of these voters are Hispanics, blacks and Chinese (the sizable Vietnamese community in Houston votes Republican), so separate at your will.