Sawmill geyser in the Upper Geyser basin. Although it only erupts to about 10-12 feet high, it's going almost all the time and is very close to the boardwalk.
Grand Geyser goes off every 12 hours, +/- 2 hours, and can hit almost 200 feet in height. It was worth the almost 3 hour wait!
Daisy Geyser, in the Upper Geyser Basin, can shoot more than 75 feet high, and erupts approximately every 3-4 hours.