Who is the best person to beat Magnus Carlsen?

Alekhin lost a world championship match to Euwe. 

Wei Yi and Jeffery Xiong are probably the only people to stop him.

Fabiano.

The cool thing about this topic is that the answer changes every few months.

Somebody with a higher ELO or autism scale. 

Very clever indeed.

Hope Levon but suspect Carauna to be the next real copetitor.

I heard, that, there's this 'dude' in our 'neighboring' galaxy, of "Andromeda".. who's a veritable 'genius'.. At numerous 'board games'.. And, is a 'quick study'..{ie. 'learner'}.

.. Even though, he reportedly knows Nothing, about what we call 'chess'.. at the moment. It shouldn't take him more than 25 seconds, for him to know, everything, {about 'chess'}.. That there is, to know! o:

You mean a blitz specialist?

Personally, I think Giri or MVL are the most likely to beat Carlsen in a match. Magnus said that he considers Aronian to be his toughest opponent. But if Giri learns to win from the great positions he gets, he would become the favorite to win the next Candidates. MVL has the technical skills of Kramnik with a unique style that makes him especially difficult to beat. 

Nakamura is a fantastic player, and fantastically strong. But he has so much to overcome, not just from a purely chessic perspective, but also from the psychological perspective that Carlsen seems to have his number. 

Maybe he has already done this, but I really think Nakamura needs to play a lot less internet chess

Naka moved into 2nd on the rating list playing blitz and bullet chess. The only person in the world who is better at internet blitz and bullet is ... Magnus Carlsen. 

Maybe all the advice of the old fogies not to play so much blitz chess is wrong? 

Sure, there are plenty of high school kids who play far too much online blitz and never improve. But chess professionals combine hours of blitz with serious study. They seem to know what they're doing.

Don't forget, chess is first and foremost a game! If you're not having fun playing chess, you're doing something wrong. And blitz/bullet is addictively fun.

That's true! And there's someone else you can add to the list of elite players who were great at Internet blitz: Mikhail Tal. I would argue that he's the greatest Internet blitz player of all time - even better than Carlsen and Nakamura! Please see my thread on the subject for a more detailed discussion on why I'm giving the title to Tal, and what others think about the subject.

https://www.chess.com/forum/view/chess-players/is-mikhail-tal-the-greatest-internet-blitz-player-of-all-time

Tal died in 1992, before the Internet Chess Server (later to become FICS and ICC) had timestamp or time seal. Back then, the 12 second increment was in place because of the terrible lag on the internet. That relic can still be seen in some of the default settings of some chess sites.

Put succinctly, Tal died before internet blitz chess was a thing.

"Maybe all the advice of the old fogies not to play so much blitz chess is wrong? "

This is a gross confusion of causation. It's their skill that causes them to be good at blitz, not blitz that causes them to be skilled. At least that's much more reasonable to say. Besides, why is Caruana so much worse at blitz than Naka if their classical ratings are so similar?

 Magnus could lose the world championship next week, we dont know. He is very well rounded in skills and he can focus for long periods without fraying at the nerves, he's also competitive to a fault which mostly works to his advantage but if someone can get under his skin that competitiveness could hurt him.

I don't think he will crack under pressure now if he didn't crack before. On the other hand his opponent is carrying the weight of all the pride of Mother Russia and that might weight in early on.

I wouldn't expect anyone to hold the world title too long in any endeavor, just like The Highlander some plucky young buck is always going to be swinging for your head and you just get older more weary every year.

Doesn't Peter Svidler enjoy a winning record against him at the moment? It may seem like a statistical anomaly but somehow I doubt Svidler agrees.

I'd probably say a statistical anomaly. Kramnik used to enjoy having a plus score, Giri did, and those have changed, as well. They're dwindling.

When it comes to the WCC, Karjakin's problem is that not only will it be hard to put Magnus into trouble, even when Magnus is in trouble, he's perhaps the best fighter around and will be extremely tenacious and hard to beat even then. In fact that's one of Magnus's strongest qualities, which you don't notice too often because he doesn't get into much trouble in the first place!

I think many depends how well prepared are players to travel on uncharted territories. Magnus beat twice Anand because he showed that, even in their classical opening combats ( Berlin, Ruy Lopez, Nimzo) the one who was ready and more comfortable to play unknown positions was Carlsen.

From the other view the same think I noticed in Sergeys recently Candidate tournament. He played variations with Black pieces in QID that was characterized dubious but he survived. He won Anand with White pieces using a rare flank attack. He generally showed that he is not afraid to change directions through the wind. The last game vs Caruana ( Siciian Rauzer ) , he could play a more safe variation that secures at least a half point, instead he played all the way.

I believe Carlsen have weaknesses to trap he's mind and Karjakin it's the ideal opponent to expose that. I would not be surprised if we have a new Champion after all this.