The Troy Times in comparing Ferguson's record with that of Dunlap, says:
"The chronic grumblers who so strenuously urged a short time ago that Mr. Ferguson, Troy's second baseman and manager, was played out-too old for active service and unable to control his men-have changed their minds, and well they might do so. Comparing Mr. Ferguson's record with that of Dunlap, who is claimed to be the best second baseman in the country, it is found that Ferguson excels him at every point..."
-Brooklyn Eagle, August 16, 1880
This has to be one of the earliest reference to Dunlap as the best second baseman in the country. It's certainly the earliest that I've seen. While the article goes on to compare Bob Ferguson to Dunlap, the important thing is not whether or not Ferguson was better than Dunlap but that Ferguson was being compared to the young Dunlap. Fred Dunlap was the standard to which all other second baseman were held. Even years after his playing days were over, baseball writers were still comparing young second basemen to Dunlap.
I think it was Bill James who wrote about Satchel Paige and said that all Negro League pitchers were compared to him. This pitcher or that pitcher was as good as Paige or better than Paige or was faster than Paige; the point being not that these men were better than Paige but that Paige was the standard to which all other Negro League pitchers were held. Paige was, most likely, the best pitcher in the history of the Negro Leagues and it was natural to hold him up as the standard. The same seems to be true of Dunlap. While Dunlap may or may not be the best 19th century second baseman, he was for some time the best second baseman in the game and, in 1883 and 1884, the best player in the game. There are many people who saw him play and believed that he was the best player of all-time and his play set a standard for second basemen that lasted until the days of Nap Lajoie.