FM335 and FM336, part of the Three Sisters, also known as the Twisted Sisters, is the best Texas has to offer. This rides best for me taking 336 north from Leakey and then take 335 south. Very scenic.
FM1431 from Cedar Park and Marble Falls is another great ride. I like riding this from east to west. It starts out wide, lame and congested and builds up to hilly and twisty. This is my go-to stretch when I need my fix.
The Devil's Backbone is a stretch of FM32 between San Marcos and Blanco. This is a popular scenic road with sections built on top of a long ridge, making nice views on both sides. This is tame compared to the above but a nice ride.
There are numerous stretches worthy of motorcycle exploration in Texas. You just need to explore to find them. There is a nice short stretch between New Ulm and Fayetteville for example.
Enjoy and Play Safe