"Something unforgettable" can be expressed in Engpsh as "something that left a lasting impression."
For example:
"I have an unforgettable experience when I traveled to Europe for the first time."
(我第一次去欧洲旅行有一件令我难忘的经历。)
"The moment I saw the Grand Canyon was unforgettable."
(我看到大峡谷的那一刻令我难忘。)