I think this one really depends a lot on how define iconic. Most of the comments so far in this thread are those truly epic moments, such as Ostagar and Anders blowing up the chantry, certainly rank there for me. Out of those kinds of scenes perhaps to me the most epic scenes is the drawing duel between Hawke and the Arishok as well as the final battle with Meredith, especially the first time when the statues started getting involved.
To me, those truly powerful and iconic scenes in DA games have been those truly emotional powerful scenes that managed to move me, with DA2, by the virtue of being my favorite in the series having most of those. Perhaps at the top of the list for me personally is Leandra's death, her last interaction with Hawke and the discussions with Isabela (LI), Gamlen and Aveline after that. There was just something truly haunting and tragic in those moments. Coming close at that list for some reason, and I am slightly surprised that this sticks out for me so much, is the return from the Deep Roads in DA2, as they stumble through and realize the sibling has the Blight and desperately try to figure out what to do.
The more I think of it, for me most of the the truly iconic moments are really small character moments that are somewhat silent yet somehow manage to speak so much of the characters. I almost started listing them here, but there would be far too many.