First of all, if anyhow possible, I recommend to stay overnight at an onsen ryokan for a true onsen experience. Daytrips can be nice too, but just don't compare to an overnight stay.In terms of quality of water, Noboribetsu Onsen is most famous. It offers has a large variety of different sources with some wonderful waters. But I also understand that many foreign visitors are more interested in the atmosphere of the town and bath more than the actual water. And Noboribetsu is not the most atmospheric resort town apart from the impressive hell valley. About Noboribetsu:
http://www.japan-guide.com/e/e6750.html
Jozankei is another hot spring resort even closer to Sapporo, but due to its proximity to the big city, it is a very developed town:
http://www.japan-guide.com/e/e6745.html
Marukoma Onsen at Lake Shikotsuko seems to be quite unique and not very far from Sapporo:
http://www.japanican.com/hotels/ShisetsuDetail.aspx?st=1535001
http://www.japan-guide.com/e/e6736.html
A very special bath in terms of water and views is found at Ryounkaku in Tokachidake Onsen, although the lodging is somewhat run down, but it is located in Furano and difficult and expensive to access in a day trip:
http://www.japan-guide.com/blog/koyo09/090916.html