[Firebird]

I personally enjoyed Sukayu onsen the most. It is set in the middle of the mountains/countryside just 1 hour south of Aomori. Sukayu Onsen is a Ryokan wich has a "Senninofuro" (the 1000people bath) wich has milky, lemon tasty water. the walls are out of dark wood and the atmosphere is great.
It is mixed bathing but because there is 1 hour a day for ladies only it is ok for both. During the usuall hours there are not many women in the bath.

I payed around 5000yen for a single room including the pick up in Aomori. I had to save money so i skipped the food wich was another 5000yen. My friends however stayed there as well and said the food i well worth it. They have a kitchen u could use!

On the way there they dropped me off (on request) on the cable car going up the mountain. From there its a 4-5 hour hike to the Onsen (u can take the bus all the way!). The hiking is one of the best ive done in Japan and its especially good during autum when all the leaves turn gold and red.

My second favourite was Akui Onsen wich is 30 min east of Sendai. Here i could stay at a busines hotel for 3500yen but use the spa of a huge spa resort hotel. The place is small and not overun, theres a beautifull waterfall a few min drive away and a nice little walk along a canyon.

I haven´t been to all the Onsen on the above posted page, but i didn´t like Noboribetsu to much as it was overrun and quit expensive (all just hotels). Kusatsu spa (a spa town) was way better and had some nice Ryokans. It also seemed not so artificial as Noboribetsu.

Shirahama Onsen is nice because it has a beach (shirahama=white beach) and some free "foot bath" just behind the beach. Nice in summer when you can watch families playing with firework in the evening.

Beppu is another nice onsen town 2 hours from Fukuoka. It has a wide range of accomodation and some socalled "hells" wich are all naturall Onsen. Some are red, some are deep blue,... best get the recommodation from the tourist office.

Most of this are Onsen towns, so if the most important is the bath, i´d definetly go for Sukayu. Also the setting is beautifull and the hike in the mountain is nice. Otherwise drop by Akui Onsen (also i dont remember the hotels name...).
For Onsen town feeling go to Kusatsu spa or some of the far off parts like Kawayu spa or Akan Kohan in Hokkaido.

Hope i could help
Firebird