Rather than hijacking the "how good are you" thread...moving my response to paultopia's Zukertort comment here:
In Elo's 1978 book, he rated the Zukertort as the 47th-strongest player of all time. Which doesn't sound like much, but he must have therefore been one of the five or ten best players alive when he was active, right?
http://www.chessbase.com/newsdetail.asp?newsid=1160
A newer rating system estimates his rating at 2700+ through his career, and puts him in the top 20 players of all time for 1-, 3-, 5-, and 9- year periods. On the five-year list he ranks ahead of five world champions (Morphy, Spassky, Petrosian, Tal, and Smyslov), and Bronstein and Keres too:
http://www.chessmetrics.com/#PeakList
Anderssen reportedly was a good friend of his. In 1883 he won a big tournament in London ahead of Steinitz, Chigorin, and Blackburne. In the World Championship match with Steinitz he blew a 4-1 lead and lost (ouch). Tarrasch believed this is what killed him.
There's apparently a very good biography (if you read Polish!). Anyway, a good english review is here:
http://www.chesscafe.com/text/review340.pdf
Here is a cool game of his against Blackburne from the 1883 tournament. He starts slow (playing the English!), then he just sets fire to the chessboard. I love the position right before the final move: White has queen and rook, black has his queen, 2 rooks, and a bishop, and is doomed.
http://www.chessgames.com/perl/chessgame?gid=1001854
Steinitz said: "Words are insufficient to express the admiration of the mastery with which Zukertort conducted this game."