n Judo, the ground game (Newaza) is given less attention than in BJJ. That doesn’t mean that Judokas are not good on the ground. There are many Judokas that have amazing Newaza and would do very well if they transitioned to BJJ. It can be argued that BJJ and Judo are the same sport (BJJ actually came from old style Judo), but a different set of rules is what makes them different.

his list of top level Judokas even has 3 BJJ black belts, and one of them is even a two time BJJ world champion.

Here are the Judokas from past and present, who have the best ground game would have to be, in no particular order:

He won the silver medal of world championships in 1975 and gold medal in 1981.

Kashiwazaki retired in 1982 and became national coach in England, Canada, Germany and so on. As of 2009, he coaches judo at International Budo University in Japan. He was knower for his superior ground game and also cross trained in Sambo and Wrestling.

From Fightingfilms.com:

Having broken his arm at the age of sixteen, he found himself unable to use his favourite technique- Morote-seoi-nage any more. He had to rethink his judo and began to study ne-waza, eventually becoming so strong that by the end of his fourth year at Tokai he was part of the team in open-weight events, due to his ability to beat bigger men on the ground.
Kashiwazaki also broadened his horizons by studying various non-Japanese forms of wrestling, travelling to many different countries in the process. In particular he devoted a lot of time to Russian Sambo wrestling- eventually winning a world Sambo title in 1975.