I will happily state that Sakura, in Komagome, was fantastic. And I believe that if you pay the 200 yen for their "member's card," you get a discount on everything else - food, massages, return visits. The discount was enough to offset the cost of the card and some more.

LapLap din't include some of the more "tourist-y" onsen in Tokyo, so I hope I'm not remiss in wondering...Note that supposedly these are onsen where they have dug deep into the ground to access hot water - are we counting places like LaQua, Ooedo Monogatari, and the Grand Shiespa/Zaboo (Roppongi)?