Lower Geyser Basin is located about 20mi (32km) from the west entrance of Yellowstone. It has the most thermal features scattered in the area. Here, you'll find mud pots, geysers, pools, hot springs and fumaroles.
It's an easy walk around the Fountain Paint Pot Trail. There were some areas I was unable to see visibly because of the steam coming from the geothermal spots.
The colors of this spring are so vibrant and the most visible compared to the others. It has a temperature of 193°F (89°C). This is one of my favorite hot spring in Yellowstone.
This place gets crowded and parking can be difficult.
I would recommend putting this as the first places to see in Yellowstone.