Although it's oil not coal, the deepwater horizon accident was probably worse than Fukushima from both an environmental standpoint and number of lives lost. The most tragic failure of a power generating facility of all time was probably the Banqiao Dam disaster, not Chernobyl.
Certainly true from a direct-deaths standpoint. There were zero killed by the Fukushima explosions, and zero acute deaths due to the radiation release. (It's a lot harder to settle on long-term chronic radiation sickness effects.) 11 were killed at Deepwater Horizon.
Not sure how you could quantify the respective environmental damage, either financially or ecologically.
But there is another effect to consider: total monetary damage. Deepwater Horizon cost BP $62 billion pre-tax[0]. The total cost of Fukushima including victim compensation is currently estimated at about $180 billion[1], but the estimate has skyrocketed over the past 6 years, so one could suspect further escalation.
