who is the worst world champion of all time

http://www.chessmetrics.com/cm/CM2/PeakList.asp
Per chessmetrics, the lowest WCh peak ratings are:
M. Euwe - #40 at 2750
R. Ponomariov - #51 at 2744
A. Khalifman - #79 at 2717
... I don't even see Kasimdzhanov in the top 100 ... so I guess he "wins," or he may be considered somehow too modern to be ranked, much like Carlsen.
As far as the worst person, Alekhine. He refused to play matches against the strongest competitors, choosing instead Bogolyubow and Euwe, and he also spread Nazi propaganda, which isn't a good look.
most of those guys werent even world champs (Kasimdzhanov, Ponomariov, Khalifman were all just there becaue of the split title when Kasparov/Kramnick was the best) ...

Of course, his reign is just starting, but my money is on Ding Liren. He's a great player, but rising tensions in the geopolitical sphere may limit his ability to prove himself in international competitions. After all, he does live in a dictatorship. We've hardly seen the guy since winning the championship.
Anyway, he barely got past Nepo in the match. And Carlsen's clobbered Nepo repeatedly. So I guess what I'm saying, slightly off-topic, is: we all know who the real World Champion is.

The standard answer to this question is Euwe, but it is still worth taking a look at the tournaments he played as World Champion. It’s not as if he was all that bad, and far from sure for example Ding Liren will do better in that respect. Euwe also won the title by beating Alekhine, which was impressive enough. And, as can be seen below, he did better than Alekhine also in the tournaments they played after the match:
The thing that struck me is that you posted a tournament that had 5 world champions in it! That may be a record?
The standard answer to this question is Euwe, but it is still worth taking a look at the tournaments he played as World Champion. It’s not as if he was all that bad, and far from sure for example Ding Liren will do better in that respect. Euwe also won the title by beating Alekhine, which was impressive enough. And, as can be seen below, he did better than Alekhine also in the tournaments they played after the match:
The thing that struck me is that you posted a tournament that had 5 world champions in it! That may be a record?
I think so. And Euwe would have been shared first if not for a horrible blunder against Lasker…
It amuses me how nobody is saying Steinitz, which is obviously the answer.
If you count only objective playing strength for example Ding Liren and Euwe are better World Champions that Steinitz, but if you count achievements in their time Steinitz was a better World Champion than most.

Steinitz faced every strong challenger in the world for decades, and prevailed. When he finally lost, it was to a man 34 years younger than him, and one of the strongest players who ever lived (Lasker). Steinitz's theories and writings are pretty much the basis for everything that followed.

Probably Liren. Because everyone knows he's not the best. He was the best player to play in the world championship, but not the best player in the world.

And when Steinitz was champion people said he couldn't compare to Morphy, Lasker admitted that in the later years of his reign Capablanca was the stronger player, everyone "knew" that Euwe's win was a fluke, Botvinnik was considered to be lucky that he kept squeaking through with ties and re-matches, "expert opinion" was that Karpov would have had no chance in 1975 had Fischer played. Kalifman, Anand, Ponomariov, Khazindzhanov and Topalov weren't "true" champions because Kasparov would "certainly" have defeated them. Bottom line, they all went through the prescribed tournaments and matches and defeated everyone. The same goes for Liren: there's no telling if Carlsen would have been able to summon up the desire and effort winning the championship requires.
”they all went through the prescribed tournaments and matches and defeated everyone. The same goes for Liren: there's no telling if Carlsen would have been able to summon up the desire and effort winning the championship requires”
Liren is the World Champion, then it is another thing if he counts as one of the greater players to be World Champion. The case against him is that he has won few events, I think he has one international top event victory in a classical round robin, after speed chess tiebreak. Being in his 30s it is not certain he will reach a higher level in the future. He has never been close to be the best player in the world, didn’t qualify for the Candidates until Karjakin was banned, and didn’t win the Candidates.
Carlsen possibly losing the title match is not something that would have helped Liren in that respect since he never could have played Carlsen in the match given that it was Nepo that won the Candidates with a big margin.
Comparing with the achievements of Steinitz, he won every match he played for 32 years, had the biggest margin ever to #2 of all World Champions according to Chessmetrics, won 25 games in a row against top players of the time, and even though the World Championship was invented first when he was 50 years old he won four title matches. He also won a match against Blackburne +7-0=0. So I think there are good reasons to count Steinitz as a great World Champion.

And when Steinitz was champion people said he couldn't compare to Morphy, Lasker admitted that in the later years of his reign Capablanca was the stronger player, everyone "knew" that Euwe's win was a fluke, Botvinnik was considered to be lucky that he kept squeaking through with ties and re-matches, "expert opinion" was that Karpov would have had no chance in 1975 had Fischer played. Kalifman, Anand, Ponomariov, Khazindzhanov and Topalov weren't "true" champions because Kasparov would "certainly" have defeated them. Bottom line, they all went through the prescribed tournaments and matches and defeated everyone. The same goes for Liren: there's no telling if Carlsen would have been able to summon up the desire and effort winning the championship requires.
Steinitz and Morphy aren't comparable at all, they never played a game, can't argue that Steinitz was better or worse really in terms of achievements because the World chess championship only existed for a part of Steinitz's reign and for none of Morphy's. Don't think we can really call Euwe's win a fluke. After Alekhine won back the title he just played politics until WWII rolled around so that nobody could contest his title (and Euwe could not recontest). Botvinnik return match in 1958: 12.5 to 10.5. Botvinnik return match in 1961; 13 to 8. It's not "squeaking through" when you comfortably win your rematches. The reason he drew/lost as a defending world champion is because he pretty much focused completely on other things while holding the title. Instead of Magnus resigning the title, Botvinnik just chose to focus on other things and then recontest as per the FIDE rules of the time. I cannot believe you would include Botvinnik on a list of worst world champions. He's top 5 in my eyes and generally put 4-6th in all time rankings. WInner of Nottingham 1936, came second at AVRO 1938, won the 1948 tournament, won two rematches, held two draws as World Champion, world champion for a total of 15 years. First won the title at age 37 and lost it for the last time at 52. Most of his prime were taken by WWII too where he was as an engineer and contributed to the war effort.
I was just pointing out the many reasons people give to "prove" that some players weren't really "worthy" world champions. I certainly agree that Botvinnik was unquestionably the world's strongest player in the 1940s and one of history's top players, but many people say that his reputation would be considerably less lustrous had the auto-rematch rule not been in effect.