1. Kasparov
2. Fischer
3. Karpov
4. Kramnik
5. Korchnoi
In my opinion, the old "greats" could never stand up to today's champions. I have looked at the games of Morphy, Alekhine, and their contemporaries, and been able to refute many of their game-winning attacks against best play. As for the new prodigies like Carlsen, they have yet to be around long enough to show us if they qualify for this rank. From what I have seen so far, though, Carlsen is not as close as many would place him.
the same is true for any sport. Boxers are faster, stronger and more technical than 10 or 50 years ago. name a sport and you will see that records get broken all the time and the depth of skills is deeper. even in the olympics the distance between 1st and 5th is fractions in many events. Its based on a skill , talent and experience. This is why you cannt really compare strengths of players from different eras. You can say that those players are talented and would have performed at a high level but thats it. Why i categorized players as who changed how chess was played or viewed instead of results.
oh…
1.Kasparov(2851 max elo)
2.Alekhine
3.Carlsen(2837 max elo)
4.Levon(2825 max elo)
5.Kramnik(2811 max elo)