Graf had a better winning percentage in 1986-96 (that is a 11-year time frame) than Seles in her best year ever (1992).
But, hey, why let facts come into the way?
After all you are a proud Selesian!!
And the fact that Graf's winning % from the ages of 16.5 to 27.5 was a touch higher than Seles' winning % when she was 18 tells us what, exactly?
When Seles was 18 in 1992, she won 3/4 Majors and the YEC and made the finals of Wimbledon. When Graf was 18 in 1987-1988, she won 2/4 Majors and the YEC and made the finals of Wimbledon & the U.S. Open. Advantage Seles.
And if we're talking about "peak" Seles, that was from the '92 U.S. Open, when she went 36-1, winning the U.S. Open, YEC, Australian Open (over near-peak Graf), and lost her one match 7-6 in the 3rd to Navratilova when she had the flu.
If 37 matches was too short of a peak, how about the 2 year period from the '91 French Open-Hamburg '93? Here are Seles' stats from that period: 128-7 (94.81%); 2 AOs; 2 FOs; 1 Wimbledon F, 1 Wimbledon DNP; 2 U.S. Opens; 2 YECs; Losses to Capriati*2, Navratilova*2, Sabatini, Graf, ASV.
That winning % is higher than Graf's winning % from 1986-1996. It is also higher than Graf's winning % in 1996. It is also higher than Graf's winning % from 1995-1996 (her "peak" according to many), when she lost to Coetzer, De Swardt, Date, Hingis, Davenport, and Novotna. It is also higher than Graf's winning % from 1994-1996, 1993-1996, 1992-1996, 1991-1996, and 1990-1996. Advantage Seles