I'm still undergraduate and want to explore various branches of physics. I just feel that I like theory more and that's it. However I'm curious which branch of theoretical physics allows you to explore fundamental aspects of universe. I've heard that String Theory, QG, HEP etc. only sound sexy and not always lead to interesting reaserch. That's why I'm asking for honest opinion - which branch of theoretical physics is really fundamental and interesting as a reaserch field?