Ding Liren’s best tournament result this far should be Sinquefield Cup 2019
How great is Ding Liren?

Nepo won the candidates back to back. Clear first. Ding never won it. He's had better results than Ding over his career. Ding might've won that rapid game but I still think Nepo is better than Ding.
Also, head to head in classical Nepo leads Ding 6 wins to 5 with 16 draws. In rapid and blitz Nepo leads 10 wins to 8 with 12 draws.
Nepo won the candidates back to back. Clear first. Ding never won it. He's had better results than Ding over his career. Ding might've won that rapid game but I still think Nepo is better than Ding.
Yes, and this is the type of result that even Carlsen at his best would have been happy with:

I find rating players from different eras almost impossible, as there is no way to compare them head to head, or having played the same competition, etc. You also have the issue of players that have lived and played in the computer era, vs those that played before computers. The differences in the advance of theory also makes a difference. So, how great is Ding? Well, as of right now he is 3rd best in the world, as his rating is lower than Magnus and Nepo. Of course that may change when new FIDE ratings are posted this month that take into account the WC tournament. Maybe his win will take him to 2nd, though the match was so close he might not see any bump in his classical rating since those games were a 7-7 tie. I don't know if the tiebreak games will count toward his classical rating.
I find rating players from different eras almost impossible, as there is no way to compare them head to head, or having played the same competition, etc
It’s a subjective business, but my own main criteria when comparing are these, in order of significance:
Time as the best player in the world
Number of top tournaments won
Number of World Championships won
Now Lasker didn’t have as many tournaments in his day as Carlsen. On the other hand he could pick his opposition in title matches, and go more than a decade between them. It’s hard to say for how long a player was the best player in the world, but one can make a decent approximation. Maybe Lasker and Kasparov reach around 20 years, Carlsen maybe 15 soon, and they are my top three.

Ding dong is the greatest chess mind of all time, may be chess writers of the next century will write about him in a way people still write about Morphy, he is the Chinese Morphy that's it plain and simple

For starters, Fischer and Magnus should be on #1 and #2. Given the importance of winning the world championship, Ding deserves a spot in the top 30 imo. Time will tell if he will grow into this title like Karpov, or if he will be considered a champ like Khalifman, Ponomariov or Kasimdzhanov. Probably something in between, as he is a strong player but not necessarily stronger than other top 10 players.

Nepo won the candidates back to back. Clear first. Ding never won it. He's had better results than Ding over his career. Ding might've won that rapid game but I still think Nepo is better than Ding.
Also, head to head in classical Nepo leads Ding 6 wins to 5 with 16 draws. In rapid and blitz Nepo leads 10 wins to 8 with 12 draws.
Yes, but some games (the last rapid game) obviously carry more weight. Do two candidate tournament wins and better overall results outweigh a world championship?
If that list of top 50 was redone today, would Karjakin still be on it?
He would probably drop out of it given that he was #49 and Ding and Nepo would be given places on it. Result wise I’m not sure he should be behind Ding though, it’s probably a bit connected to popularity. Drawing a title match in classical against Carlsen is hardly less impressive than doing it against Nepo, but Karjakin also won a long list of top events that Ding never won. For example Candidates, World Cup, Norway Chess (twice), Tal Memorial, Wijk aan Zee, Dortmund, etc.
The list spans different centuries, how can we compare Ding to Morphy, they solved completely different challenges, it's not possible to compare their strengths and weaknesses.
The match I would had liked to see is Ding-Carlsen , probably Carlsen would have an edge there but he doesn't want to play WC chess anymore, which is fine, people should do what they want.
Carlsen withdrew the right way, he said it openly and didn't try to find an excuse nor did he try to taint the next champion, who may not be at his strength but won the title fair and square.
Ding is an amazing player and in this WC match he produced two masterpieces. Some of his games over the years are textbook quality for how an active positional player should play and create winning chances.
Personally I see Ding as a more active version of Karpov, don't know how they'd compare directly in terms of strength though, peak Karpov was incredibly strong.
Also, we probably have not yet seen peak Ding, this will likely happen at some point over the next 10 years.

He is world champion, and a top player.... How good we shall have to wait and see. But I would bet against him nearing Magnus achievments.
Also, we probably have not yet seen peak Ding, this will likely happen at some point over the next 10 years.
Such things are always unpredictable. When Karjakin and Caruana took Carlsen to playoff in their mid 20s few would have thought that they would do much worse when they were 30. I think it is rather rare for players nowadays to peak when they pass 30. Carlsen is 32 and his best results are probably in the past. Caruana is 30 and may also never improve on his results from his mid 20s. But Ding could well peak much later than them.
Also, we probably have not yet seen peak Ding, this will likely happen at some point over the next 10 years.
Such things are always unpredictable. When Karjakin and Caruana took Carlsen to playoff in their mid 20s few would have thought that they would do much worse when they were 30. I think it is rather rare for players nowadays to peak when they pass 30. Carlsen is 32 and his best results are probably in the past. Caruana is 30 and may also never improve on his results from his mid 20s. But Ding could well peak much later than them.
I don't know if something has changed recently, 2-3 examples don't disprove it, a wider sample is needed but it used to be that players typically peaked between 30 and 40
Also, we probably have not yet seen peak Ding, this will likely happen at some point over the next 10 years.
Such things are always unpredictable. When Karjakin and Caruana took Carlsen to playoff in their mid 20s few would have thought that they would do much worse when they were 30. I think it is rather rare for players nowadays to peak when they pass 30. Carlsen is 32 and his best results are probably in the past. Caruana is 30 and may also never improve on his results from his mid 20s. But Ding could well peak much later than them.
I don't know if something has changed recently, 2-3 examples don't disprove it, a wider sample is needed but it used to be that players typically peaked between 30 and 40
It's certainly very different between different players. Leko dropped quickly after 25, while Korchnoi and Anand both were very strong in their 40s. For some reason all Chinese players have peaked unusually early, Bu Xiangzhi, Wei Yi, Wang Yue, Yu Yangyi, Li Chao, Hou Yifan, Ni Hua. All of them had their best results when they were very young. Wei Yi can of course still improve, but he was higher rated when he was 15-16 than now, eight years later.

Ding may not be as established as some of the other players, but he has some impressive classical accomplishments of his own (to add to his World Champion title).
Was a 3-time National Champion of China (youngest player ever to win this - had a 2800+ performance rating when he was only a 2400-rated player). He went undefeated in all 3 of those championships.
He won the Moscow Grand Prix, ahead of Nakamura, Giri, MVL, and Nepomniatchi, with a performance rating of nearly 2900. Was undefeated in that tournament, as well.
Also won the Sinquefield Cup by beating Magnus 2 games in a row (in tiebreaks). It was Magnus's first time losing a tiebreak in over 12 years. Ding went undefeated in that tournament, too.
Ding and Carlsen are also the only players to each go 100 consecutive classical games undefeated at the top level ...
Many of these accomplishments happened recently, within the past few years.
We'll have to see where things go from here ... but it seems like he's still rising.
”Ding would be somewhere between maybe 5 and 20”
I think 5 sounds high. The list above has Karpov as #6 and then he was #1 for a decade and top two for more than 20 years.
Karpov was the exact reason I was tempted to say 5 at first. Karpov, like Ding, won the world championship only when the best player in the world declined to defend the title. There's an argument that Karpov was never actually best in the world. And while Karpov was excellent at not losing, Ding shows strong signs of being better at winning.
I think you underestimate Karpov a bit, he could score results like this, in Linares 1994, long after his peak