I tend to see tennis history in three great eras:
"Prehistory" - starting around 1860 and before 1877; development of lawn tennis from various other games.
First Era - Starts in 1877 with the first Wimbledon.
Second Era - Starts in 1926 with the beginning of the pro circuit. Pro/amateur split era.
Third Era (Open Era) - Starts in 1968 to present.
Wilding was probably the greatest player of that first era, or at least until Bill Tilden came along in 1915 (although Tilden wasn't truly dominant until 1920 or so). Though the eras were quite different, he was about as dominant in his prime (1906-1914) as Tilden was in the 20s/30s, Gonzales in the 50s, or Laver in the 60s.
Using TennisBase's rankings (which I haven't had access to for a few years as they started charging exhorbitant prices), here are the players who they ranked #1 for at least three years:
13 Tilden
8 Laver
7 Wilding, Gonzales
5 L Doherty, Sampras, Federer
4 Ritchie, Budge, Lendl, Nadal, Djokovic
3 Barlow, Pim, Riggs, Rosewall, Connors, Borg, McEnroe
As you can see, it is a pretty short list and only two players had more years than Wilding. Of course given the size of the pool, the more recent the more each year is weighted. Regardless, Bill Tilden is also very underrated, perhaps partially because of his rather sordid proclivities.