This is kinda like having to chose your favorite child. So here are a few.
1) early morning just as the sky was lightening, I was hiking alone in Wa. All the sudden the ground started to rumble. It was a large herd of elk. I could catch glimpses of them through the trees but the power was extraordinary.
2) Oregon, middle of the afternoon. A huge owl (later confirmed as a great gray owl) glided right over our head and landed in a nearby tree. It would let out a tweet and then a short distance away two babies would answer. Definitely one of my favorites.
3) near Reds Meadow (Sierra) in the middle of a huge burn area we were walking in intermittent snow. Sudden Wrongturn stopped at pointed at the ground ahead of us. There were three newborn fawns that were perfectly still. They had their eyes closed as if saying "I can't see you, so you can't see me!"