There are so many, and I'm still finishing the game. Obviously, the Fall of Haven/Skyhold sequence was uber epic, and very personal when it came to saving people in camp. There are still a couple I regret and miss when wandering my keep.
Like many others, my first experience with a dragon was the Frostback in the Hinterlands, and while it didn't outlevel my group too badly, the landscape, the dragonlings, and just the feeling of the battle that this was a DRAGON, not to be trifled without peril. I died several times, and finally, bleeding and limping, my party finally managed it down. It was tremendously more satisfying than my experiences in Skyrim, or even fighting the Archdemon back in DA:O. Since then, I have completed the Dragon Hunter achievement, and while there were some difficult and challenging fights, none have gripped me as that first encounter.
I will critique somewhat about the differences from DA:O, but the interactions and banter between the companions is extremely fun to see again. And I'm impressed as hell so far with some of the personal companion quests. I care about my group, and how they feel about my character. That's always been one of my favorites about Bioware games.
Adamant Fortress, talking down Clarel and trying to save as many Wardens as possible. As my character points out, these were heroes who had saved the world countless times to a debt that could never been fully repaid. To oppose them was harsh and painful, remembering the struggles of my Hero of Ferelden, and the efforts done as Commander of the Grey. But evil could not be allowed to win their hearts and souls.
I could go on, but I'm gonna go play Inquisition some more