MOST consider her tops.
On which surface is Graf not among the top-2 ever?
Clay?
Grass?
Hard-Court?
The reasons I put those 3 above her are the following:
Court: longevity, total achievement and doubles dominance (I dont regard doubles that highly but when you completely dominate in doubles and singles both for a decade that is different)
Navratilova: peak level of play, total achievement, and doubles dominance.
Evert: consistency, longevity, total achievement.
As for the various surfaces it is hard to compare Court and Evert on hard courts to Graf as they didnt have near the opportunities to play on hard courts. Court I would say is similar to Graf on each of grass, clay, and indoors. Evert is superior to Graf on clay, inferior on grass, similar indoors, and similar on decoturf. Navratilova is superior to Graf on grass, superior indoors, similar on decoturf, probably would be inferior on rebound ace, and inferior on clay.
All 4 are very close but Evert and Martina have won between 155-170 tournaments each, and Court according to people who have researched it has won close to 200. That along with Evert and Court excelling first as teenagers and then well into their 30s, and Court and Navratilova completely dominating doubles as well, is why I rate the great Graf just below those 3 even if ability wise she might well be as strong or better. I am considering their careers and each aspect of it.
I do believe had it not been for all her injuries Graf would easily be the greatest ever today (including without the Seles stabbing). Unfortunately her injuries prevented Graf from fulfilling her maximum potential as a player, as amazing as her career already is.