Aetna is the most active volcano and most spectacular in Europe outside of Iceland. But the potentially most dangerous is the Campo Flegrei caldera west of Naples. Heavily populated urban areas at risk, and the whole region is still smoking from the last one around 500 years ago. Last report there was increasing uplift signifying rising magma.
I read several years ago that Campo Flegrei has so much magma under it if it ever has a large eruption it can take out Naples.