It's impossible to say definitively which of Albert Einstein's scientific achievements was the *most* important, as each had profound impacts on physics and our understanding of the universe. However, his Theory of General Relativity is often considered his most significant work due to its revolutionary nature and lasting influence.

Here's why:

* Expanded on Special Relativity: General Relativity built upon his earlier Special Relativity, which revolutionized our understanding of space, time, and gravity.

* Unified Gravity and Spacetime: It showed that gravity isn't a force, but rather a curvature in the fabric of spacetime caused by mass and energy.

* Explained Phenomena: It explained phenomena like the bending of light around massive objects (gravitational lensing), the precession of Mercury's orbit, and the expansion of the universe.

* Foundation for Cosmology: It laid the foundation for modern cosmology, allowing us to understand the evolution and structure of the universe on a large scale.

* Continued Relevance: General Relativity remains a cornerstone of modern physics, providing the framework for understanding gravity and its implications in various astrophysical phenomena.

While other achievements, like his photoelectric effect and mass-energy equivalence (E=mc²) were equally groundbreaking in their respective fields, General Relativity's profound and lasting impact on our understanding of the universe makes it a strong contender for his most significant scientific achievement.