For skills, tactics, experience, mentality --- Steffi is certainly the best of all.
The only thing she could be lacking is strength and speed.
In fact, tennis has gone through a lot of changes ever since The William Sisters emerges.
I think skills and tactics still count a lot like when we see Hingis making back to the Top10 when she returned.
The Prime of Steffi, Hingis vs Prime of Wil Sis, Davenport, Maria.....etc. can be close matches since they all represent different styles of gameplay.