Focusing on the main quests here:
In Hushed Whispers had a pretty big impact on me. It was powerful to witness that possible future and the whole quest had a really sorrowful, hopeless feeling to it. I also found Alexius quite an interesting and even tragic villain. Also, that ending where the music picks up, your companions decide to sacrifice themselves, Leliana goes berserk mode... I got freaking goosebumps. Really great introduction for Dorian too.
I also agree with WobbleLikeAJelly that In Your Heart Shall Burn was really captivating the first time I played it. The music, the visuals and the atmosphere all worked well together. First proper introduction to our villain too. This quest is the one which made me really pumped up about all the possible things to come. (Too bad they really didn't live up to my expectations.)
When it comes to least favorite... well...
Many people seem to like it but I found Wicked Eyes and Wicked Hearts.... tiring. The premise was great but I thought the execution was rather clumsy. The hassle with the halla statues, the timed exploration and some downright sillyness like the Inquisitor leisurely climbing a wall while in clear view of the other quests. Or characters who are supposed to be veterans at playing the game (Leliana, Vivienne) practically doing nothing while the Inquisitor solves everything, no matter how ill-fitted for it from a roleplaying POV. That brings me to my other issue with the quest: there's a clear-cut "right way" and "wrong way". Your ability to RP is hindered since a failure just makes you restart, rather than actually having interesting consequences.
I don't feel like this because the quest would be hard or something. In fact my very first Quisitor got the highest court approval. I just think they could have done more with the whole "court intrique" theme since I somehow failed to feel it and found its' mechanics pretty unimpressive and tiring instead.
Still, I don't hate the quest and there's some elements/scenes I really like about it but generally I was still rather unimpressed and in each playthrough with a new character, I'm not really looking forward for it.