My mother an I visited the 9/11 memorial this past spring, and the the best I could come up with was "heavy."
I thought hard about this very problem you are asking, and while "memorable," and "impressive" did come to mind, most words seemed to lack something. I don't have a huge command of the English Language, but this was my break down:
Awesome - No
Cool - No
Memorable - How? No emotion, just that I will remember it, goes without saying.
Impactful - Ok, ish. Doesn't feel right though.
Impressive - What? Thousands die, careful with that word.
Moving - Ok.
Intense - A lot of things can be Intense, what kind of Emotion?
Heavy - To me this wraps enough of the above up, without excitement.
Heavy is strong (Intense), deep emotional impact (not surface level, internal)
To me it meant there was a lot to take in, thought provoking, and touched me deeply.
My thoughts. But it really depends on how it affected you, personally. Without knowing the feelings it drew up in you, it is difficult for us to suggest a good descriptor. You do not want to use someone else's word to communicate a feeling you had. Look inside of yourself, and really think about it. Skirt from things that you feel don't fit.
Don't dwell on the words that you can't use, find the word that you feel you must use.
What helped me was to step through words, start with the first word that comes to mind, then a word like it, then like that, and so on, thinking about the quality of the word and what you like or want to avoid in it to take you to the next word. It took me a good few days to figure it out for myself. Also, it is important to note that one word that works for one monument/memorial, may not work for the next.