I grew up in DFW but spent a lot of my childhood in Austin and San Antonio. I've been everywhere in Texas except the valley.
Houston has the busiest freeway in the state (US 59) and Dallas has the second busiest (US 75). DFW has more lane miles but also more people than Houston's metro area. Both cities have a lot of big ticket stacks and lots of lanes. Fort Worth has a much older freeway system than Dallas (older as in roads that have not been touched since the 50s) and still has a few two-by-two laned freeways (parts of 820,35W,287,etc) so I think the congestion there is often worse. Dallas beats Houston only by virtue of an extensive rail system that is actually used.
Overall, though, I'd say San Antonio has the best freeway system in the state - now that 410 is done. For smaller cities, Corpus and Waco both have really good networks.
I always thought El Paso sucked, but I dont have a ton of experience driving there. I think the terrain and the layout of the city makes it unwieldy and gives motorists less options than, say, Lubbock would.