The Drover in Fort Worth is quite impressive. Not "most luxurious" but definitely in the "most special" category. See my review in that thread. The suite we got was great and the grounds are just really well kept.

The Ritz and the Mansion (now a Rosewood) in Dallas are typical luxury properties.

Not Marriott and just outside your 5 hour zone (unless you drive 85, which you probably reasonably could), the St. George in Marfa is one of my favorites. Get a nice suite, enjoy the pool and the generally cooler weather compared to the rest of Texas.