DM here. The biggest thing for me is when the players get emotionally engaged to the story and the beats. They fell actual hatred for the BBEG. They get so excited about a reveal that they all can’t talk for a minute. Something happens in game, and half the players are having a chat in the Discord in character, outside of game time. The reactions from the players are so rewarding.
The next best is when they come up with a solution to a problem in the game that I hadn’t even considered. It’s kind of fun to have to just say “OK. Hang on a minute. I was totally not prepared for y’all to try that, so I’m going to need to figure out what they impact of that is.” The players always get really excited about that; I think they love to stump me.
High tier is not so much about how hard something is to do, and more about the choices they have to make. For example, my L18 party is about to have choose between cutting the legs out from under the upcoming ilithid invasion, thereby saving millions of lives, or saving the paladin’s beloved homeland.