adjective. Earning a permanent place in the memory; memorable.

adjective. Not forgettable; enduring in memory.

adjective. Which is very difficult to forget

adjective. impossible to forget

"I'll keep a list of these things - of the things that people have said that they are looking for in a book they would describe as "unforgettable" - next to my computer, so that I can see them while I write, so that I can be reminded of what I am reaching for."

forgettable  

haunting  memorable  persistent  red-letter  

forgettable  regrettable  resettable  

bizarre  breathtaking  compel  electrify  evocative  

celebrated  conspicuous  distinguished  eminent  esteemed