What is the most troublesome optical aberration?

The most troublesome optical aberration is often considered to be spherical aberration. This occurs when light rays striking a lens near its edges are focused at different points than those striking near the center, leading to a blurred or distorted image. Spherical aberration can significantly degrade image quality in optical systems, particularly in Photography and telescopes, making it challenging to achieve sharp focus across a wide field. Mitigating this aberration typically requires specialized lens shapes or additional corrective elements.