Littlefoot's injuries from the fight with Cera, but not for the physical damage. But, Drake, you may say, the point is to pick the event you believe hurt the most. Well, I have.
The fight with Cera results in the rest of the gang abandoning Littlefoot and following Cera on the wrong path. Maybe they would have followed her even if Littlefoot won, but I think seeing how tough the three horn is helped to sway them. Also, Littlefoot is clearly broken up when Cera takes the wrong path and he's unable to stop her. The delivery of the line "Fine! Go the wrong way!" is so strained, I think the idea of her going off to get lost and maybe killed and his own inabillity to stop her had Littlefoot choking up. The rest following her is just another slap in the face for him.
This turn of events gets the rest of the gang into terrible trouble and then moves the shame and humiliation unto Cera when she realizes she should've listened to Littlefoot in the first place! Sure everything works out in the end, but this one little event splits the gang up and could even have resulted in everyone dying, if Cera had shown up at the end to help deal with Sharptooth. Therefore I pick this scene for the emotional pain it inflicts on Littlefoot and Cera as well as the audience when they see the two heroes arguing and fighting.
*Builds Fortress of Smugness behind Wall of Text and stands triumphantly.*
