In Memory of Flowers.
When we first meet Aerith, her sole responsibility was protecting the flowers in the church. On several occasions throughout the series (and in more than one game), she makes a point to inform intruders, whoever it may be, not to step on the flowers. Although it shows her caring nature for even the smallest and most delicate of life, she also makes it her duty to protect them. This carries onto her duty as a Centra living in the Lifestream; regardless that the lives of the planet may seem small and in millions, she still whole-heartedly watches over them. Even in death, she still appears to her friends and speaks to them as if she never left, guiding them through hardships and troubles.
In the case of Zack, he tries his hardest to become a "hero", though this very goal simultaneously blinds him from what it really means. Ironically, he was too busy searching for the answer to his dream though the answer was closer than he expected; Aerith tends to and cares for his flowers right in front of them. Throughout his travels, he finally realizes in the end that being a hero is not rooted in pride or skill, but in the capacity to care and watch over everything most important to him. At the end of Advent Children Complete, Cloud and Denzel come across a patch of flowers covering the place where Zack died. Denzel asks if it is a grave, and Cloud says no, it was a place where a hero began his journey. The double meanings imply the start of Cloud's story and the ending of Zack's; it is the beginning of two heros. Zack dies while protecting Cloud, finally becoming the hero he had wanted to be, and knowing that he made a particular girl in his life happy for spending time with him.
The flower wagon that Zack and Aerith creates is one of the most memorable scenes in Crisis Core in the sense that regardless of how extraordinary they may seem, they still have the capacity to lead normal lives and do normal activities. Zack had agreed to help create a wagon for Aerith so that she may cart her flowers around the slums to sell. Zack, being the more confident type, tries to convince Aerith to sell them on the upper plate, believing that she will do exceptionally well. Moreover, he explains that flowers are a rarity in Midgar, and wishes for Midgar to be full of them instead. This very ambition opens up a new door for Aerith: in later years to come, her flower-selling business becomes successful, and she is able to ascend to the upper part of Midgar, as well as the rest of the world, without fear.