Jomon no Yu

This Fuchu spa complex was the dearest onsen in all of Tokyo when it first opened in 2006, charging bathers a gobsmacking ¥4,300 for a dip, though the prices have been reduced to (slightly) more sensible levels since then. Expensiveness breeds exclusivity, of course, and you won't have to jostle people aside for a space in the baths here. The salt-rich onsen waters used at Jomon no Yu are said to prevent sweating, meaning that you won't feel a sudden chill when you get out of the tub, and on a clear day you can see Mt Fuji from the men-only rotenburo bath on the top floor.