If I had to list the one thing on my trip that was most impressive I’d easily cite Yellowstone’s Grand Prismatic Spring. It’s a massive hot spring (3rd largest in the world) running with rivulets of color. Even the spring’s mist seen at a distance is chromatic. It really is astounding to behold and how can you not like something with such a pompous title?
I also have a great tip on how best to see it (provided by the aforementioned park ranger). After you’re finished walking around the spring and seeing the adjacent Turquoise Pool and Excelsior Geyser Crater, go find the Faery Trail behind the spring. A short distance down this trail is a small mountain where you can easily climb up and get a sweet arial view of the spring and really get some great pictures. You can then continue on the Faery Trail to see the Faery Waterfall if you like.