I'd have to say my favorite is Austin. It's the most memorable both man made and geographic. Always excited to just be there an explore.
After that, I would have to say Houston. I never thought, before moving here, that I would say that considering Houston's image, but it really is a melting pot of so much worldly detail. It's got its little disfunction, particularly a cohesive walkability factor, but that's something that a planner like myself can promote and change. It's my favorite city to drive on the freeways (in non-rush hour of course).
San Antonio and Dallas are on the same level for me. I love both their downtowns. Dallas has a neat organic feel to it with a splash of modern infrastructure and San Antonio is just a preservationists heaven. It took me a while to warm up to downtown San Antonio, but the time I went there during the work week made me appreciate it so much more, locking into a comfortable liking of the place despite the tourists...but that's a necessary component to SA's likeability, I suppose. I have to wander around the rest of the city a bit more, because from what I've seen, I've been pretty creeped out.
I also liked Corpus Christi. I wasn't sure what to expect when I went there the one and only time, but I connected with it pretty well. I like the corridor from downtown to the beaches. And the stroll along the downtown waterfront is very enjoyable and decently connected to the loads of potential the city streets offer.
I haven't been to Ft. Worth yet and that's my next priority.
Overall, I've been pretty impressed with Texas cities, but they do take some time getting use to...especially Houston. It gets on my nerves many times still!