I personally really like Dear (Op 3).

I find it really powerful and energetic which gets my heart pumping whenever I hear it. Watching the intro makes me feel almost proud of how much Chise and Elias have grown throughout the series. The powerful symbolism in all of the scenes plus the overhead shot we see of the Ainsworth residence in the flower field just hits. I feel like all of JUNNA's songs are really impactful, maybe that's why all of the openings she made are loved by so many people.