In my case that would be scenes which stir up emotions rather than action scenes. For example the goodbye scenes with Chomper, Ali, Mo and Tickles (though the last one was not so important a character his goodbye scene was great). Or sometimes scenes with an impressive landscape picture (like when they are finding the water trapped in the mysterious Beyond in LBT 3 or when they are overlooking the land of mists in LBT 4). Or when Chomper apparently feels at a loss after his parents were driven back by his friend's parents in LBT 2.
Very often the scenes that I like most are accompanied by "If we hold on together". So how about you?