you can never decide who the best is..However these were the champions..
Morphy
Steinitz
Lasker
Capablanca
Alekhine
Euwe
Botvinnik
Smyslov
Tal
Petrosian
Spassky
Fischer
Karpov
Kasparov
Kramnik
Anand
Topalov
Carlsen
That's a very bizarre list.
How is it that Morphy is considered champion but not Greco, Philidor, LaBourdonnais, McDonnell....
How is it that Topalov is considered a world champion but not Kasimdzhanov, Ponomariov, or Khalifman?
Personally, I wouldn't include either Morphy or Topalov in a list of match-play World Champions. That line goes from Steinitz to Alekhin, then from Botvinnik to Fischer and then from Karpov to Carlsen. Carlsen is the 16th match-play World Champion.
Rightly said
As I recall, Fischer was about 100-150 Elo ratings points better than Spassky after the Taimanov, Larsen, and Petrosian matches of 1971. This was, I think, the largest margin ever between numbers 1 and 2 in the ratings.
In 1972, Fischer carried around with him "the red book" - a collection of Spassky's games.
https://www.youtube.com/watch?v=7ZLrcafDtkg
I have no doubt he had all of Karpov's published games at hand, and thoroughly dissected.