What should I specialize in?

Civil Engineering, as you may know, has a variety of sub-disciplines: Structural, Environmental, Construction, Materials, Hydraulics, Geotechnical, and Transportation (at least that's what is at my Uni) and I am severely on the fence about which one to pick for potential grad school.

A little background: I am a junior expected to graduate on time with a 3.85 GPA and I'm working for a structural engineering professor as a research assistant. I have the national center of asphalt technology in my backyard. I don't have any idea what kind of job prospects are out there.

Sub-disciplines eliminated because I'm not absolutely fascinated by the subjects: Construction, Hydraulics, Environmental

Subjects I really like: Geotechnical, Materials (Asphalt and Concrete), Structures, Transportation

TL;DR: I'm considering grad school. What sub-discipline of Civil would put me in the best position to have success?