In 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.

This 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:

Katsuhiko Kashiwazaki (Japan).

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.

Udo Quellmalz (Germany).

Udo Quellmalz os 1996 Olympic Champion and 1991 and 1995 World Champion. He was performance director of the British team and coached in Austria and Qatar. Double Olympic medallist 1992-1996, 4 World Chs. medals and 3 European medals.
He was revered for his aggressive style and dominant ground game and particularly for his Quellmalz choke.

Neil Adams (Great Britain)

Neil Adams won numerous Olympic and World Championship medals in judo representing Great Britain. Adams was the first British male to win a World title, and the first British male to simultaneously hold a world title and a European title. Other achievements include a gold medal at the 1981 World Judo Championships in Maastricht, the Netherlands, plus silver medals in the 1980 and 1984 Summer Olympic Games and the 1983 Judo World Championships. Adams was also five-time European Champion. On 20 September 2008 he was promoted to 8th Dan at the age of 49. He is known for his ground game (ne waza) and his arm lock.

Flavio Canto (Brazil)

Flávio Canto is a Brazilian judo champion and Brazilian Jiu-Jitsu black belt, who won the bronze medal in the – 81 kg division at the 2004 Summer Olympics in Athens, Greece. He previously competed at the 1996 Summer Olympics in Atlanta, Georgia. Canto won three medals in a row at the Pan American Games: 1995, 1999 and 2003. He is known for his high level ground game and there are many stories of him having dominated many BJJ champions on the ground in training.

In 2004’s there was talk of organising a grappling superfight between Flavio Canto and Ronaldo Jacare (then world bjj champion, who also had a strong Judo background). The project never came to life.

John Baylon (Philippines).

John Baylon is one of the most decorated judo players ever to come out of south east asian region, with a record of 9 SEA games gold medals (the highest in a single medal sport) he has twice represented the Philippines at the Olympics
and been a medalist in the worlds masters, asean and asian games.
In addition to his judo skills John is also a black belt in brazilian jiu jitsu under clube de jiu-jitsu in japan. John is the first person from the south east asian region to hold a kodakan judo black belt and a bjj black belt.
John been both a national bjj and multiple national open judo champion. In 2008 in a Judo tournament in the Philippines, Baylon famously submitted Pan Am champion and famous BJJ black belt Mike Fowler by Ude Gatame armlock.