Both are common and acceptable in casual speech. -1- "It is a really unforgettable experience" is weak because unforgettable is an ungradable adjective. - The meal was good. (correct) - The meal was very good. (correct) - The meal was really good. (correct - here really good is a synonym for very good) - The meal was unforgettable. (correct) - The meal was very unforgettable. (weak - very unforgettable doesn't work) - The meal was really unforgettable. (weak - really unforgettable doesn't work) -2- "It is really an unforgettable experience." This is an emphatic statement, often used to contradict an earlier statement. - I heard Paris is not as wonderful as it seems in the movies. - No, no, no! Paris is really an unforgettable experience! We enjoyed the Louvre, the Eiffel tower, and the wonderful food!

Both are correct.

yes。could you tell me why？

It is really an unforgettable experience i thought