After a long time of playing, I have come to the conclusion that Ruy Lopez is one of the best, if not the best opening; perhaps the greatest most complex openings out there. I respect this opening as it was proven that if white plays the lines correctly, he will carry a small advantage. Its also a great opening since it explains develop and castle early and build a attack. It is my signature attack.
The Sicilian defense would be the most complex, and rather annoying. This opening was discovered and analyzed in 1604 extending to the early 1800s.