Formal/Strong:
* Flawless: Emphasizes the absence of any errors or imperfections.
* Perfect: The absolute best possible, leaving no room for improvement.
* Exemplary: Serving as a model of excellence, setting a high standard.
* Unsurpassed: Superior to all others, unmatched in quality.
* Faultless: Without any flaws or mistakes.
* Immaculate: Perfectly clean, spotless, and free from blemish.
Less Formal:
* Spotless: Perfectly clean and free from dirt or stains.
* Top-notch: Of the highest quality, excellent.
* First-rate: Very good, of high quality.
* Superb: Exceptionally good, outstanding.
* Brilliant: Exceptional in intelligence or talent.
Specific to different contexts:
* Precise: For accuracy and meticulousness (e.g., impeccable/precise instructions).
* Elegant: For refined taste and style (e.g., impeccable/elegant dress).
* Unblemished: For moral integrity or reputation (e.g., impeccable/unblemished character).
The best substitute will depend on the specific context and the desired tone of your writing.