Kurokawa Onsen in Kumamoto Prefecture
The first onsen I ever visited in Japan was Kurokawa Onsen in Kumamoto Prefecture. Although I've visited many other onsens in the 19 years I've been in Japan (and they're all nice!), my favorite is still Kurokawa Onsen. Why? Because the view is FANTASTIC! The rotenburos (outdoor hot spring pools) at most of the inns are built right on the small, beautiful river that is the heart of Kurokawa onsen. The river is surrounded by lush, refreshing greenery and is really a treat for the eyes.
Kumamoto prefecture is also well-known for its beef, so you're bound to have a good meal at any of the inns.