15, 18, 23, 26 of the inquisitor headcanon meme for... anyone you finished the game with
The Temple of Mythal, most definitely. Seeing the Emerald Graves and Exalted Plains was awful for him, but stepping into the Temple of Mythal was unbelieveable. I don't know if there was anything more stunning to him than seeing something built by the elves he descended from--the culture he, and his entire clan, were clinging to desperately.
If there had been more time, he would have wept, I think.
18. What will they do after they defeat Corypheus?
Part of me thinks he'll try to rebuild Kirkwall, and make it a place of healing and good deeds, to try and make up for all the bad that has happened there--and to give Varric his home. HOWEVER in my canon, the Lavellan clan ...gained a fucking city. They have a home now, so he might want to spend some time there, with his clan and his lovers, letting the Inquisition run on its own for a while.
23: Do they side more often with Leliana, Josephine, or Cullen? Why?
Josephine, mostly because he got most of his advice from her and he trusts her.
26. Name one to three things that you headcanon about your Inquisitor that is not/cannot be confirmed through in-game choices.
He rarely speaks to anyone he doesn't already know. Even to people he knows and loves, he goes through periods of selective mutism where he just stops talking. The Inquisitor is incredibly verbose, so it's hard imagine him deferring to one of his companions basically every time someone speaks.