I like Electronics, that specifically include Radio-Frequency circuits and Embedded processors, with a fair bit of computer science and networking application. I have recently become a fan of artificial-neural-networks. I love to see algorithms come to life with electronics and computer science merging to form Robotics.
I am always quite curious and excited about Astrophysics with topics like Parallel Universe, Time Travel, Einstein's Laws, String theory, Unification of General Physics and Quantum Physics. I also find it interesting to understand consciousness, its 'materialistic' definition and if it can be transferred from natural-neural-nets (Brain) to artificial-neural-nets.
The best part of all the above is Mathematics, it's embedded in each and every one of them. It's the joy of seeing equations come to life and their interactions with real-life objects.