Obviously, there are spoilers in this article through the end of season 3 and roughly book ... 4? Just about.
From the first episode, Summer has been on point. He whimpered warnings from the ground as Bran made his fateful climb up the Tower of Brotherly Love; he ripped the throat out of the assassin sent to kill Bran, then immediately lounged on a fur blanket in what I can only assume is the direwolf equivalent of a postcoital cigarette; and he shared Green Dreams with an imprisoned Bran while roaming the Winterfell Godswood. But at the end of Season 3, he dropped the ultimate “I’m a Better Pet Than You Are, So Take That, You Silly Fire-Breathing Dragon” trump card, letting Bran realize his full warg potential, enter his mind, and share his skin in order to kill a group of approaching Wildlings. Even Hodor’s penis had to be impressed by that.