I reviewed the book Ground Zero: A Reassessment of the 1917 explosion in Halifax edited by Alan Ruffman and Colin D. Howell which is a pretty detailed comparision by historians and scientists of 130 conventional explosions which was presented at a scholarly conference: a good example of a wikipedia verifiable and reputable source. They still ranked Halifax as the biggest by an overall measure (power, radius, lives lost, destruction) but did conclude measuring such explosions is difficult. I have summarized their conclusions in the article with footnotes and links.     