The tallest active geyser in the world is Steamboat Geyser in Yellowstone National Park. Some of its eruptions blast water as high as 400 feet into the air. Don't go to Yellowstone expecting to see it erupt because it has erupted fewer than ten times in the last twenty years.
Waimangu Geyser in New Zealand used to be the tallest geyser in the world. Its eruptions were spectacular, blasting jets of water up to 1600 feet in the air. Unfortunately, a landslide altered the hydrology around Wiamangu and it has not erupted since 1902