The most radioactive object in Chernobyl, The China Syndrome. 1997. 320×301.

The elephant's foot is often regarded to be the most radioactive piece of material in Chernobyl Unit 4. Meet; The China Syndrome.

The Elephant's Foot is treated as if its the worst piece of radioactive material on planet earth when, the peak official measurement is 8,000 roentgens per hour. Corium at Fukushima has measured over 20,000 roentgens per hour. Nuclear fuel rods can be even up to 50,000 roentgens per hour.

It also is not even close to being the biggest or most radioactive object or piece of corium inside Unit 4. It is only famous because it was the first to be found, and everyone went "ooo scary solidified radioactive blob" as it was reported to the media while other findings were not published. If we assume everything is proportionally radioactive, using radiation figures taken on similar dates, the Upper Heap in 012/15 would have measured about 10,000 roentgens per hour when the elephant's foot was measuring 8,000.
The most radioactive, a GIGANTIC LFCM covering an entire corridor in the 210 steam distribution levels, The China Syndrome, would at its peak be measuring around 14,000-18,000 roentgens per hour at one of the steam outflow drums in 210/7 , when the elephants foot was found. It also reaches over 10,000 in several other rooms. Take these numbers with a grain of salt however, as they are estimations.

So, what is The China Syndrome?

It formed of course shortly after the explosions where it pooled into the room 305/2 OTM +9.0, directly beneath the reactor. Large amount of corium separated and went East into what is known as the great horizontal flow, including the elephant's foot. Our corium, went down, into the large vertical flow. As it burst the pressure membranes in the floor of 305/2, it traveled down pipes intended for the emergency discharge of steam, and flowed out the steam drums in the Steam Distribution Corridors of 210/7 and 210/6.
The most radioactive of these is seen in Photo 1, coming out of the most southwesterly of these drums.

Not much is known about its discovery other than the complex expedition found it, a wall had to be dug through to reach it, and it was found long after the discovery of the Elephants foot. It is noteworthy for being the largest and most radioactive mass, about 10x as large as the foot by Volume, and weighs 230 tons. It also has an average uranium content higher than the peak uranium content found in samples of the elephant's foot. The China Syndrome name only came into use a few years ago when it appeared on a website by Ppitm where it was popularized. He says the name is supposed to represent how it is the vertically flowing corium, like the China Syndrome movie.
It would likely be far more radioactive if Concrete was not pumped through these corridors in 1986.

The image depicts the Most radioactive part of "The China Syndrome." 3460 Roentgens Per Hour in 1997, meanwhile the Foot, had 700, around the same time. Located in 210/7.