There isn’t one universal “best” aperture, because aperture affects both image quality and the look of the photo.

General rule of thumb:

• Wide open (for example f/1.8): lenses often show more aberrations and can be a bit softer.
• Stopped down a little (often around 1–2 stops from wide open): many lenses are at or near their sharpest.
• Very small apertures (such as f/16–f/22): diffraction softens the image.

So yes, many lenses perform best somewhere in the middle of their range, but the exact point varies by lens design, focal length, and camera. Diffraction also depends on sensor/pixel size; on many APS-C cameras it can become noticeable around f/11 or so.

That said, “avoid” is too strong a word. Every stop has a use:

• Use wide apertures when you need more light or shallow depth of field.
• Use small apertures when you need more depth of field or a slower shutter speed.

In practice, if you want maximum sharpness, start in the mid-range and test your specific lens. If you want a particular creative effect, use whatever aperture gives it—even if it’s not the sharpest setting.