Which extinction event killed the highest proportion of organisms?
This is a great question. I decided to research it and here is what I found. For some reason the GOE is not on lists of the "big 5" mass extinctions. The question is why?

I think it's due to the pace of the event. The event is described as happening 2.45–2.32 billion years ago. 



This is a timescale in billions of years. So it isn't really a proper mass extinction. Little is known about the time scale of the evolution of cyanobacterial lineages.