Probably the most spectacular hot spring in Europe is in Iceland. Of course this country has natural spas everywhere and most of them are amazing, but the place that impresses visitors most is the Blue Lagoon. This is a huge hot spring with the highest rate of natural water flow in Iceland, and is now part of a resort where visitors go for relaxation and other spa treatments as well as the springs themselves.
Almost equally famous is the Geysir area to the south, where you will find a number of hot springs which can produce a column of water going up almost 200 feet. There is also a volcano in this region, which has been occasionally active again since 2000.
You can see a full list of Iceland's hot springs here.
Other European countries have hot springs well worth visiting too. Bath in the UK has a well known natural hot spring, and so does Matlock in Derbyshire. Southern Germany is also famous for its spas especially the area near Baden-Baden. You can find a list of other European spa destinations here.