There isn’t a reliable brand-wide rule. Lens quality varies a lot by individual model, so you should compare specific lenses with the same focal length, aperture, and features rather than assuming Nikkor is always sharper than Tamron or Sigma.

In general, Nikon makes some excellent lenses, especially at the higher end, but Sigma and Tamron also produce very good lenses—and sometimes offer unique options Nikon doesn’t. All of these brands also have weaker entry-level lenses, so brand alone doesn’t tell you enough.

If your priority is optical performance, read reviews and comparisons for the exact lenses you’re considering. Depending on the model, sometimes the Nikon version is best, and sometimes a Sigma or Tamron is just as good or a better value.

One practical difference noted by users is resale value: first-party lenses often hold value better over time than third-party lenses. So if long-term investment or resale matters, that may favor Nikkor even when image quality is similar.