It's the clearly bit that I and many others object to.
Since FIDE ratings have been introduced, there have been 14 WC matches (not counting aborted matches and split titles). In 3 of these the outcome was different than that predicted by ratings. Furthermore, in one case the winner (Kramnik) was 70 rating points below the loser.
By contrast, the peak rating difference between Carlsen, Kasparov and Fischer were 97 and 66 points respectively.
And there's one additional element. When Kasparov retired, he was 41 years old and 47 points below his peak. Given his age, it's highly unlikely he would have beaten his own record had he continued to play. Carlsen too seems to have peaked, oscillating 40-60 points below his peak for the last 4 years (but since he's only 27 he has better prospects of going above 2882 one day).
Fischer on the other hand was 29, and had dropped a mere 5 points below his peak... hardly a sign of decline.
All in all, perhaps not as clearly cut as you picture it...
Simple: you have a pool of more players and you play only with higher-rated opponents, so even a draw would up your rating.
In the old days, the players' pool was narrower, but also more democratic, meaning that world champions frequently played also much lower-rated opponents, in different events, which is NOT quite the case now, excluding rapid, blitz and exhibitions.
Fischer was the best, there is no doubt about that, Kasparov's play is more GRANDIOSE, but Fischer's is more CORRECT.
More correct means higher elo.
Grandiose play is often mistaken for stronger play, but that is simply not so.
This has more to do with the style of the player, attacking play/wins certainly impress more.
I have checked Fischer's play in his later career with SF and he seems to make very FEW tactical mistakes, even in the most complicated of positions.
Simply incredible.
I could not believe SF sees the precise lines Fischer plays, which almost no human would think of in
a range of positions. Precisely the computer line.
Which is not true of Kasparov.
For example, his games against Karpov from 84-85, where he got the title, are full of tactical inaccuracies.
Another thing to consider is that Fischer played much stronger in quieter surroundings.
How stronger would he have been under such conditions, +100 elo, more?
So, no doubt, Fischer is the best, we will see what Carlsen does in the future, for the time being he is just competing.