Answers might vary from reader to reader on this question, but I think that one of the most important items in the series is the wardrobe that allows the four Pevensie children their initial access into the world of Narnia. The wardrobe itself was made from a tree that grew near where Digory buried the magic rings from The Magician's Nephew. The rings are important because they function similarly to the wardrobe. They grant access to Narnia.
I would say that anything that gives access to Narnia has to be considered important, so the list should include the picture of a ship at sea, the gate in the wall, and the railroad junction. I would also say that the Pevensie's initial entrance into Narnia is one of the most pivotal moments in the series as the Pevensie children are central to multiple books in the Narnia series. Without the wardrobe, Lucy and her siblings would not have been able to meet Aslan and defeat the White Witch.
The Pevensie children are able to battle the White Witch and rescue Narnia from the White Witch's power because they are given special items to help in the fight. Peter is given a sword and shield, Susan is given a bow, arrows, and a magical horn, and Lucy is given a small dagger and magical healing cordial.