You should say: where you saw or heard it; what kind of advertisement it was; what the contents of the advertisement were; and explain how you felt when you saw or heard this advertisement.
口语高分范文：
The unforgettable advertisement that I would like to tell you about is a famous one called "NBJ". I really think it is very creative. As far as I can remember, I watched this advert on a video website named Youku when I was surfing the Net. My friend told me that it was chosen as one of the best advertisements in China last year.
This advert is considered to be a successful commercial. For example, theproduct "NBJ' became a household name just because it shows the exaggerated excitement of people when they get it. So these words were on everyone's lips. In fact, it got to become a bit of a bore.
Finally, I would say this advertisement left deep impression on me. Maybe my personal experience can help you to understand. It was a cold winter's afternoon. I was watching this advertisement online as I mentioned before. I heard the slogan being repeated several times which grabbed my attention. It said "we only want NBJ in the Spring Festival". This became a bit of an earworm so that after watching it, you still remembered the name and found yourself mindlessly repeating it to yourself. I was totally amazed by the effect of this advert. Yes, it was really an unforgettable advert.
高分口语词搭：
creative 有创造力的
commercial 商业广告
a household name/ a household word 家喻户晓的名字
leave an deep impression on sb. 留下深刻的印象
