In 2015, an international team of volcano experts (called the Global Volcano Model Network) released a report (linked below) ranking the countries most at risk from volcanic disasters - both from the volcano itself and from the hazards wrought by the volcano. The thought was to increase awareness for the most at-risk areas... to urge monitoring and emergency planning. More than 278,000 people have died in volcanic eruptions since 1600, and only five eruptions caused 58 percent of recorded fatalities. Of the total deaths; 33% were killed by pyroclastic flows, 20% by tsunamis and 14% died in lahars. Only 887 people died from lava. Finally, the remaining 24% of deaths were indirect, of famine, disease, ash, avalanches, lightning and other hazards.
Thus, this is a difficult list to make. Not because there aren't volcanoes that are dangerous, there are. Volcanos are doing their thing around the globe every single day... around 50 at a time in-process of erupting, getting ready to erupt or just releasing gas and steam. Most of the 1,500+ currently active volcanoes are nowhere near human populations so they don't make the news, but others are so intensely monitored that if they hiccup any nearby town, city or port flies into action.
Do you rank a volcano's potential danger by how much activity it's starting to show even if the nearby town has all the plans in place? Or by an inactive volcano's potential for catastrophe if, despite no indicators and minuscule odds, it did go off and wiped out half a continent? Different types of volcanoes also erupt in different ways - some ways more catastrophic than others. A good example is Kilauea which has been active for some time on a populated island. Is it dangerous? Most definitely, but, because it's a shield volcano and because of the composition of its magma, the slow moving lava doesn't pose a deadly threat. So, it's active and it's dangerous, but should it be ranked higher than Mt. Rainier? Mt. Rainier does not seem to be in danger of erupting any time soon, but if this stratovolcano did suddenly spring to life, the resulting pyroclastic flows and lahars would be catastrophic for the many millions of people living within spitting distance of it. Should it rank higher than an actual erupting volcano?
For these reasons, I am not putting ranked numbers on this list, but I will loosely group them - in my amateur opinion - by most dangerous at the top down to dangerous-but-not-as-dangerous at the bottom. I am sure there will be disagreement... which is why no one should use this list as any kind of scientific resource.
Lastly, I am not putting supervolcanoes on this list because the odds of those big boys going off any time soon are miniscule.
* Decade volcanoes were determined by the International Association of Volcanology and Chemistry of the Earth's Interior to be a volcano that exhibits more than one volcanic hazard (people living near the Decade Volcanoes may experience tephra fall, pyroclastic flows, lava flows, lahars, volcanic edifice instability and lava dome collapse); shows recent geological activity; is located in a populated area (eruptions at any of the Decade Volcanoes may threaten tens or hundreds of thousands of people, and therefore mitigating eruption hazards at these volcanoes is crucial); is politically and physically accessible for study; and there is local support for the work.