Fed has the H2H lead on grass against everyone.
I'll try to find Fed's winning percentages on grass so we can compare with candidates from other ERA's.
That should be a huge indicator.
Edit:
Found this site with winning percentages however it doesn't include older records which are needed.
I also haven't checked the sites accuracy.
http://www.tennis28.com/slams/winpct_onesurface.html