B1
Something that is unforgettable is so good, interesting, etc. that you remember it for a long time.
Dictionary examples:
The unforgettable moment when I called my teacher 'Mom' still haunts me. (0.0)
The teacher's unforgettable haircut made students laugh for weeks. (0.0)
His unforgettable snoring made the whole camping trip a nightmare. (0.0)
Seeing Niagara Falls was an unforgettable experience. (23.0)