What was the most significant moment in the history of Robin's character on How I Met Your Mother?

The character of Robin Scherbatsky, played by Cobie Smulders, had several significant moments throughout the 9 seasons of How I Met Your Mother. However, one of the most memorable moments was in the episode 'Symphony of Illumination' which aired in season 7.

In this episode, it was revealed that Robin cannot have children due to a medical condition. This was a significant moment in Robin's character development as it showed her vulnerability and added another layer to her personality. It also impacted her relationship with Ted, who had always wanted children, and eventually led to the end of their romantic relationship.

Overall, this moment not only added depth to Robin's character, but also contributed to the overall storyline of the show.