As far as I am aware the highest radiation dose
Naturally the `record' must be limited to the subset of known cases. I've been studying the history of Soviet nuclear science and industry for a few years. Things went on in the Soviet Union that beggars the imagination, as they say.
When the waste storage tank blew up in Mayak in 1957, 90% of the high level waste fell in the immediate vicinity. That's 90% of 740 PBq (740E15 decays per second) within about half a kilometer radius, in which there were certainly some number of workers, this being the most urgent period of nuclear weapons development.
There were criticality accidents at Mayak that killed people as well; the Review of Criticality Accidents (2000) mentions seven incidents at Mayak and eight at other Soviet sites.
Then there is Chernobyl. Shortly after the explosion soldiers on the grounds of the plant policed up pieces of graphite and other debris, including fuel, from the reactor core with simple tools, bare hands and no respiratory protection . They were breathing particles of heavy metal isotopes so "hot" that they floated through the air on their own thermal output like little balloons. They were treated as military casualties and their numbers are not publicly known.
The worst case of radiation exposure took place in the Soviet Union. We do not know the circumstances, how severe it was, how many it killed, when or where it happened, but that it did is a metaphysical certitude.
1. The Legacy of Chernobyl, 1992 Medvedev