Leaving out asteroid impacts such as the one that is thought by some to be responsible for the creation of the moon, I’d have thought that Volcanic incidents are the most likely candidates.
In power terms you may have to narrow this down since Basaltic flow volcanoes are the biggest on these terms but wether they qualify as explosions is rather fuzzy, there are immense lava sheets in Siberia, India - possibly the most recent at around 65 million years old, Australia, South America, North America.
These are likely to be responsible for mass extinction events, as an illustration there was a far smaller Basltic flow event in Iceland(but still the largest in human existance) called Laki, which changed the Northern hemisphere climate for decades afterwards.
Here is a link
Toba in Indonesia was bigger still but homo-sapiens were not around then though their ancestors were.
There was a huge and very explosive event in New Zealand, Taupo that has been dated as being something almost within recorded history(around 5000 years ago) and might be a possible candidate.This is quoted by many of the sites I have visited as being the largest in human history-as an explosive event.
There have been much larger events still, Yellowstone Park is effectively one ginormous cladera and when it went off around 600million years ago is reckoned to have been in the order of 1000 times more powerful than Mount St Helens, methinks that is a very likely supect.
[Barbera Windsor innuendo) Ooer, guv, that’s ever such a[/Barbera Windsor innuendo] big one