The "hardest" engineering branch can be subjective and depends on your interests and strengths. Here are a few commonly cited challenging branches:
1. **Aerospace Engineering**: Involves complex systems, high-level mathematics, and a deep understanding of physics.
2. **Chemical Engineering**: Requires knowledge of chemistry, thermodynamics, and process design, often with a focus on practical applications in industries.
3. **Electrical Engineering**: Covers a wide range of topics from circuits to signal processing, often requiring strong problem-solving skills and a good grasp of mathematics.
4. **Mechanical Engineering**: Encompasses thermodynamics, fluid dynamics, and mechanical design, requiring a solid foundation in mathematics and physics.
5. **Nuclear Engineering**: Involves complex concepts related to nuclear reactions and radiation, which can be quite challenging.
Ultimately, the difficulty often comes down to what you find personally challenging and engaging.