Super Peace Busters are friends forever. Alternatively, the Super Peace Busters should always get along. Both are equally heartwarming, though.
Jintan's Big Heroic Run in Episode 11, carrying Menma on his back so she can see everyone before she disappears.
When Jintan, Anaru and Poppo all accomplished their goal with the Nokemon games. At the very least it was a sweet, nostalgic moment for anyone who's said, "Gottcha Catch 'em All~!" when they were kids.
The series finale, which, while ambiguous, hinted at the ship teases eventually getting together, and everyone in general being able to move forward in their lives and be happier and healthier.