If you describe something as unforgettable, you mean that it is, for example, extremely beautiful, enjoyable, or unusual, so that you remember it for a long time. 令人难忘的
adjective [原级]难以忘怀的,令人难忘的
A visit to the museum is an unforgettable experience. 对该博物馆的参观是一次难忘的经历。