Looking over this Rybka 3 analysis of players by era is fascinating. Looks like he completed this around 2009, with Kramnik 1998-2002 being the most recent player evaluated. Basically he compares the moves a player made and compares it to what Rybka 3 thinks is the best move. He then compares their "error rates" to modern players of various elo ratings. He also adjusts for things like opening knowledge IE he doesn't start tracking until after move "X" with X being a bigger number the more modern a player is to reflect opening theory. He also attempts to adjust for time controls used in the various eras.
All that said, here's the average expected error by thinking time chart.
Deep Blue, 1996-1997
Kramnik, 1998-2002
Karpov, 1986-1990
Fischer, 1968-1972
Capablanca, 1916-1924
Fine, 1936-1940
Average 2700 elo player
Anand, 1997-2001
Kasparov, 1989-1993
Korchnoi, 1977-1981
Smyslov, 1954-1958
Botvinnik, 1945-1949
Spassky, 1967-1971
Petrosian, 1960-1964
Keres, 1955-1959
Nimzowitsch, 1927-1931
Alekhine, 1929-1933
Tal, 1958-1962
Marshall, 1912-1916
Tarrasch, 1894-1898
Rubinstein, 1910-1914
Maroczy, 1903-1907
Euwe, 1934-1938
Pillsbury, 1899-1903
Reshevsky, 1951-1955
Average 2500 elo player
Lasker, 1892-1896
Chigorin, 1894-1898
Morphy, 1857-1859
Zukertort, 1880-1884
Steinitz, 1882-1886
Average 2300 elo player
Average 2100 elo player
A couple of interesting things. No one else in Morphy's era was even worth including in the list apparently. He truly was a peer without equal. A GM caliber player against amateurs by today's standards. A lot of that has to do with the romantic style of play that was favored at the time; attack wildly at all times to prove your manhood.
Tactical style players like Kasparov and Tal don't fare as well overall. He includes stdev, and when you take them at their absolute best, Tal and Kasparov end up #1 and #2 ahead of Deep Blue with ratings in the low 2900s. Deep Blue, Korchnoi and Spassky are the only others that hit 2900 in that metric. One comment you can make about that is strong tactical players are worse overall, but they try to make the game more complicated to give themselves an advantage.
Korchnoi's got royally screwed by his government. He had to throw his match against Petrosian for the right to challenge Fischer in 1971. Then after Fischer resigned his title, the government didn't allow anyone to help Korchnoi to prepare for his match against Karpov for the title and still made it a close 12.5 to 11.5. He had to defect at his peak to even have a chance to compete for a world title, but in doing so was basically on an island as far as support went. There weren't a lot of strong GMs outside of Russia to help him prepare for a match against Karpov, but still made it a close 6-5.
Seeing Capablanca in the top 5 overall, yet being from the WWI era is amazing. If Morphy was the 1st modern GM, Capablanca was the 1st super GM. Reuben Fine is the best player on this list to never win a title. It's a shame WWII happened as he certainly could've beaten Alekhine at that time especially right after the AVRO tournament.
Anand is considered to be the most tactical style player on this list, by a wide margin.