My absolute favorite moment in the 10 years of playing was obtaining "Broken War," both in how it happened and the progress it symbolized in the story and as a player. I happened to be using my original Warframe Excalibur at the time, and it made the moment feel so much more powerful!
That's where I awakened, but now I'm mainly using my favorite Warframe, Lavos. I love learning the history of him and the evolution he went through, and it inspires me to keep adapting to changes going on in my life.
Thanks for 10 years of amazement!