Which volcano worries you most / what will be the next significant eruption?

[LAKISYLVANIA]

My vision

The most dangerous volcanoes that are capable of very large eruptions are during our lifetime (VEI 7+)
Laguna del Maule (Chile)
Iwo-Jima (Japan)
Okataina (New Zealand)
Serdan-Oriental (Mexico)

There are others that are also dangerous, but they're not likely to erupt (Long Valley, Yellowstone, Toba, Taupo) or too small (like Katla, Sakurajima, Rainier, Agung, Ilopango, ...) while having regardless very dangerous circumstances in certain situations (near populated areas). A volcanic eruption doesn't have to be very powerful to kill or bring disruption.

The island of Iwo Jima is about twenty metres higher than it was in 1945 due to a growing magma chamber underneath. The beach where the American forces landed in 1945 is now 17 meters above the ocean surface. The island has been pushed up by 1 meter every 4 years since several hundred years. It is only a matter of time before the whole island explodes. Although few people live on Iwo Jima itself, a large eruption would cause a tsunami that could devastate southern Japan and coastal China including Shanghai and Hong Kong. The team estimates that in Japan the tsunami could be 25 meters high. The eruption of the similar sized Kuwae volcano in Vanatua in 1458 caused a tsunami 30 meters high in northern New Zealand, and lead to the cultural collapse of Polynesia.

[Frodo]

There's a possible supervolcano lurking underneath the Aleutian island chain that might be worth keeping an eye on:

Alaska islands may be part of single, massive volcano
Preliminary analysis of a far-flung island cluster suggests that what we thought were several small, independent volcanoes might actually be a single eruptive behemoth.

Though this likely belongs to the 'extremely dangerous but not likely to erupt in our lifetimes' category. 

[dead0man]

3 million Italians live under Mount Vesuvius


The Yellowstone caldera is the one most dangerous to me, but it's not frequent.  Any 500,000 years now.

[LAKISYLVANIA]

3 million Italians live under Mount Vesuvius


The Yellowstone caldera is the one most dangerous to me, but it's not frequent.  Any 500,000 years now.

Mount Vesuvius is dormant now. It hasn't erupted since 1944, but the volcano cycles between active phases and less active phases.

Between the 217BC and 79AD the volcano didn't erupt. Before 217BC, the volcano was in a phase frequent but less active eruptions, after that it went dormant for 295 years, but the next one is the famous and destructive Pompeii eruption. It was preceded by a damaging earthquake in 62, and I suspect modern monitoring would have been able to give proper warnings at time.

Since than the volcano was never quiet for such a long time, but it became very quiet around the 13th century. It erupted violently again in 1631 starting a new phase of frequent but relatively less powerful eruptions. Since 1944, it's been dormant again. From what I know i believe this won't change soon. I believe the volcano is preparing for another "Pompeii-magnitude eruption. But I believe it won't be during our lifetime. And if it will, it will be less violent than when it waits a few more centuries.

The Pompeii-similar eruption before Pompeii was around 2400BC. There was also another one around 7000BC and more rougly around 10000BC, 14000 BC and 16000BC. Some were even more powerful than Pompeii. All of them seem to share the same charasteric of having a centuries-long period of quietness. But volcanoes can change.

It just seems like it won't do anything soon, but it also seems like the next one will be a very big one.

Another type of concern is that they (Naples) literally live on top of a volcanic complex that should be capable of supervolcano eruptions, but there are no signs it will do something major anytime soon. The last VEI 7 eruption probably contributed to the extinction of Neanderthals, and ash of it was found in Kazakhstan (around 37.000BC)

[LAKISYLVANIA]

The Yellowstone caldera is the one most dangerous to me, but it's not frequent.  Any 500,000 years now.

The thing is it's about time Yellowstone erupts. But it could easily wait another 100.000 to 200.000 years to do something extremely major because of the timescale we're speaking off. Also between those intervals, there were smaller, non-supervolcanic eruptions (but also very infrequent). To be honest, it just seems it's not there (yet). We would know on time before it erupts extremely violent again. I think it's very unlikely to happen in our lifetime. It could be tomorrow on a geological scale, but that could still take 5000 to 10.000 years.

Yellowstone has another problem. It's moving under tough rock hostile towards eruptions (or it's not the hotspot moving, but the North-American plate), under a craton (arch-landmass). There are very few eruptions recorded under cratons (almost none). It's possible we have seen it's last major eruption already, but It could also do one or two more supervolcanic eruptions, before the characteristics of Yellowstone will change. There's a possibility eruptions might even be more infrequent but also much more dangerous (flood basalt eruptions)

That being said we have no / little ideas of how hotspots behave under cratons. The Siberian traps (which is the biggest eruption, since the rise of complex life) which happened 250 million years ago, happened under a craton. Almost all life on land went extinct (99%). In the ocean, around 90% went extinct. That extinction happened around the same date of the Siberian traps. It was much more severe than the extinction which killed the dinosaurs. It took around 30 million years for ecosystems to fully recover, but the Early Triassic had a harsh and very dry climate with high CO² levels. This is why dinosaurs and avian creatures have such an advanced breathing system, while precursors of mammals stayed very small and mostly adopted a nocturnal scavenging lifestyle.

It's unsure what happened, but there are suspicions the hotspot that caused the Siberian trâps is the same one that created Iceland. The antipole of the Siberian traps at the time also strangely with an enormous crater in Antarctica (Wilkes Land Crater), but it's not clear whether it's an impact crater or not. It's four to five times bigger than the Mexican crater that supposedly killed the dinosaurs. It could also be volcanic of origin. It also happened just before Antarctica and Australia split, and perhaps an impact weakened the crust there that much it caused Australia and Antarctica to split. But all of these are theories.