My favorite moment by a slight margin for this show is their #9. I might be in the minority here but the Viper/Mountain fight scene as one of if not the best one-on-one fight scenes ever, in TV or film. So that'd be my #1.
#2 - I'd go with the Red Wedding for all the reasons you said.
#3.. Danny speaking her mother tongue. That was really cool and a defining moment for her character.
#4 - Hodor holds the door.
#5 - Jon Snow meets the Night King. That was great.
I would have to watch the show and take notes for a true top ten as I'm sure there are many moments this list left off that I thought should be here. A few that come to mind - Brienne fighting the Hound. Or Arya being chased and then killing the Waif.
I do agree with Cersei bombing the Sept. That would make the list for me for sure.
Oh, what about Cersei's 'SHAME' walk?