Galveston, Corpus Christi, and South Padre Island.
Of the three in order of best beaches is South Padre, Corpus Christi, and last Galveston.
However, Galveston has many historic homes open to the public and is worth the trip. South Padre has the best beaches. Corpus has good beaches also. Both have excellent seafood. If you just want the beaches go to South Padre or Corpus. If you want a lesser beach but like history and tours of old mansions, go to Galveston. None of the three will disappoint.