Carlsen is not helping his cause by his refusal to play in the WC.
He's in the candidates tournament in March though.
He was in the previous one too until Nov 2010, but I don't think he has been given a reason to withdraw again.
Sultan Khan.
I'm glad to see I'm not the only fan :)
It was certainly a spectacular 5 years.
I came here to say Korchnoi and am not disappointed with the comments so far. Kasparov is also a good choice, if a bit obvious, because of the amount of time he spent at the top, which notably bridged the non-computer and computer eras.
Here's another reason why Kasparov is a good choice -- He's active outside of Chess. FYI, consult the (cerebral) article below by GM Ken Rogoff, discussing Kasparov's latest endeavors and debates.
Emanuel Lasker. World Champion for 27 years, author, mathematician, philosopher. Decades ahead of his time. Look him up. At 67 he was still turning in world-class performances.
Never heard of the guy.
Capablanca.
Botvinik. He mentored Kasparov, Karpov and other Russian great chess players. He also was a world champion several times.
Scottrf -- perhaps. I think in my mind "best chess career" does involve positive influences. After all, one could say Stalin and Khrushchev's contributions to chess were fairly significant, but would we really hold them up as icons. So when I think about Kasparov, I agree that he was a great player, Yes, but that is balanced in my mind by his personally responsibility in destroying the WCh cycle, devaluing the WCH title, and so forth. His lasting legacy contribution to chess outside of his games is, I think, questionable.
Is he a great chess player? Absolutely. I think he's probably rightly considered the greatest player of all time. But I don't think his career as a whole is anything to brag about because I don't think you can simply ignore the negatives and say "greatest player ever, 20 years at the top, full stop" as if the rest never happened.
Absolutely! Kruschev may well be the greatest man of the twentieth century.
I really like the Botvinnik idea. I don't know why I overlooked that. What a career arc.
Well, Botvinnik might have won the title 3 times, but he also managed to lose it 3 times :)
I'll put the cat amongst the pigeons, and nominate Sam Loyd.
Never heard of the guy.
Exactally what I was going to post :p
"Look him up." hehe.
Carlsen has time. He got the all-time record FIDE rating this year. And if anyone suggests it is inflation, I'll find the research that shows it's not.
Carlsen probably has a much healthier attitude to chess than Fischer, who clearly had no idea where to go after winning the WC. Perhaps that precedent is one reason not to take part. Or maybe not.
Here's another reason why Kasparov is a good choice -- He's active outside of Chess. FYI, consult the (cerebral) article below by GM Ken Rogoff, discussing Kasparov's latest endeavors and debates.
http://www.project-syndicate.org/print/technological-stagnation-and-advanced-countries--slow-growth-by-kenneth-rogoff
I see this as clear evidence of his lack of credentials outside of chess. His misguided ideas are only being noticed because he used to be the WCC.
I do not care what sport you are playing!
When they ask "Who had the Best career"?
They respond "Who had the most championships"?
Nothing esle matters!
And Kasparov is just another (partly) Jewish WCC taking on the Russians??
@Elroch's comment above (post #37) applies (equally well) to another WCC discussed ad nauseum in these forums.
Neat trick, @Elroch. You prove once again that "pinheaded" chess players dump will dump willingly on former WCC's.
You don't have to agree with Kasparov's political railings against the Russians in order to admire the guy, or his outstanding chess record. Ditto with Karpov. Both are "giants" of the game, with records to prove it.
Always time for another cheap put down? @Elroch's lazy-ass, one-line dismissals are his stock in trade. 
I do not care what sport you are playing!
When they ask "Who had the Best career"?
They respond "Who had the most championships"?
Nothing esle matters!
There's a reason that in the Pro Football Hall of Fame you will find folks who weren't players. Careers revolving around a sport or activity are not limited to those who engage in the activity. And indeed, very rarely is it a person who directly engages in the activity at the highest level who does the most for it.
Sports writers, organizers, sponsors, coaches, owners and so forth do a great deal for an activity. Khalifman won a championships, IM Malcom Pein didn't. But do you really think that Pein's "London Classic" doesn't have the potential to be a far more noteworthy accomplishment than Khalifman's FIDE crown?
Do you really see Ruslan Ponomariov's career as superior to Bronstein's or Keres' or Korchnoi's or Reshevsky's or Nunn's because he won the championship and they didn't?
The Champion is here!!!!
Do you remember the contenders after 20 years?
When they mention Dynasty they want to know who was the champion!
When they start looking for most valuable playey,
Who do they start look at the first? The champion(s)!
When the want to sell a product related ... they seek the champion first.
When the season or league is over who goes on the talk show .....
again the champion!
Who do kids want to most like emulate THE CHAMPION!
When they approach a champion they want to see the hardware! Go tell the players in the Olympics that did not even get the Gold but several Silver and may have more medals than the gold medalist who had the better career.
They most likely trade all of those 2nd places for 1 Gold Medal to be ........... THE CHAMPION!
When those 3 girls competed for 2 spots in gymnastics in London 2012 for best over all and the one that was the defending champion could not make the final round was hot as than a cat on a tin roof. Tell her that being the back to back champion would not improve her chance of being the best ever! A little on "Back to Back Champion"!
When the New England Patriots went undefeated and lost the Super Bowl tell me what happen! A bunch of silence and crying. "The Giants just shook up the world" The Champions, i bet the take that championship over that undefeated season, mainly about the players that never won a championshiop on the Patriots.
Let's give Magnus Carlsen time. It's be pretty cool if he became another long time, legendary giant in the Fischer/Kasparov ilk though. But that's getting ahead of ourselves.
BTW, for the football analogy-QBs who have championships: Trent Dilfer, Rex Grossman
QB who doesn't: Dan Marino
Of the three, players in the HOF: Dan Marino, only
So I guess because he didn't win a championship he had a worse career than Dilfer and Grossman, right?
Carlsen is not helping his cause by his refusal to play in the WC.
He's in the candidates tournament in March though.