Mount Vesuvius is probably the most dangerous volcano in terms of the overall risk and likelihood of eruption within the reasonably foreseeable future. This is because it tends to erupt explosively and the slopes of it are densely populated with people. You will probably have heard of its famous eruption in 79AD when the nearby cities of Pompeii and Herculaneum were destroyed. It has been silent since 1944 but is by no means inactive.
If you’re talking about the solar system, the most volcanically-active planet is Jupiter’s moon, Io, which is literally being torn apart by gravitational tides from Jupiter itself. Io’s volcanoes erupt sulphur and the poisonous gas sulphur dioxide and its lava is the hottest known at about 1500 degrees C. The largest volcanic eruption in the solar system occurred on Io in 2001.