When it comes to describing something that is not to be forgotten, there are several synonyms that can be used. Some examples include memorable, noteworthy, unforgettable, outstanding, remarkable, exceptional, and significant. These words all indicate that something is worthy of being remembered and that it stands out from the ordinary. Whether it's a momentous occasion, an achievement, or an experience that leaves a lasting impression, using any of these synonyms can add emphasis and importance to what you are describing. So, whether you use 'not to be forgotten' or one of its synonyms, conveying the idea of something unforgettable can help make your writing more powerful and engaging.