Speculating about what might have been, and comparing chess and chessplayers of different eras is a fun diversion within the chess world, as it is in many other games and sports. Here are some of my humble opinions. I would like to hear from readers, especially where they think I'm full of it...
Of course, the best players of today are the best players of all time--they're 'standing on the shoulders of giants,' after all. Kasparov (when he comes back?), Anand, Topalov and the other greats of today have studied and absorbed everything that came before, and used the latest discoveries in openings, training methods and psychology to improve. A more interesting comparison is how the greats of all eras stack up to the rivals of their day, and their contibutions to chess outside of just winning. Finally, today's stars still have many achievements (and disappointments) ahead--it's much easier to be sure about those players whose careers are finished. Some opinions:
The Geniuses (the 'Mozarts of chess'): Morphy, Capablanca, Tal, Fischer, Kasparov
Longest in the top group: Lasker 1894-1936
Most dominant for a period: Alekhine 1927-34, Tal 1957-60, Fischer 1967-72, Karpov 1977-82, Kasparov 1989-1995.
Greatest Overall Career: Euwe. World Champion, winner of major tournaments, author of many important books and FIDE President.
I'm ready for differing opinions. Let the discussion begin!
(And check out Chessmetrics for stats to back up your arguments).