I had seen pictures of the Grand Prismatic Spring in Yellowstone NP and when I got there, the view from the boardwalk was a little underwhelming, but I wasn't about to give up. After some research I found out what looked to be a better vantage point and went back the next day, on the other side of the spring, on a hill with a little bit of off-path scrambling required. While the view of the whole thing is a spectacular sight, the moment I pulled out the 600mm from my bag I saw that this was the photo I wanted.
As a side note, I recently learned that since I visited Yellowstone at the time of this photo, the NPS has built an overlook so that you don't have to climb off the trail to get to this view