Are we talking about profit, awareness level, or what?
Financially I don't think it's anywhere near the "biggest" musical hit.
And when you count all the hundreds of productions of something like The King and I for example, plus all the licensing fees I can't see Wicked even being in the top 10.
When adjusted for inflation it would probably rank even lower.
And I don't think the awareness factor is nearly as high as the classic shows.
Basically it's a popular show, but I don't think it's had the cultural impact that many other shows have had.
The Wizard of Oz brand is so huge that I think Wicked is considered just another synergized off shoot. I know that it's completely separate in regards to rights ownership etc., but public perception is such that it gets lumped in with other Oz related media which dilutes its individual impact.
....but the world goes 'round