There are lots of candidates from throughout the history of tennis, not just during the last 25 years. For example, Judy Tegart Dalton, Helga Niessen Masthoff, Olga Morozova, Wendy Turnbull, Rosemary Casals, Andrea Jaeger, Zina Garrison, Dinara Safina, Kay Stammers Menzies Bullitt, Dorothy Head Knode, Beverly Baker Fleitz, Elizabeth Ryan, Lili de Alvarez, and Eileen Bennet Whittingstall. ("For example" implies there are others, too.) That's why your poll is so lame and myopic.
Well there you have it. The best of all-time not to win a major. The one we all know and love:
Eileeen Bennet Whittingstall!
Call that Hall of Fame! Time to erect a new wing in honor of the late, great, well almost great Eileen!
Goes to show you how tough it can be to win a major. And how luck certainly plays a part. And how I still say those who did play, and win, majors in the 1970s, like Baroness Barker or Mima Jausovec, or Babs Jordan, don't have to be worried about being on the list of greatest who DIN'T! Cuz they DID win a major. Others like Mrs. Dalton, Casals, Turnbull, Ryan, Sukova, Dementieva, et al have not won a major, even if their careers rate higher than some of the winners like Ruzici, or even Miss Myskina Majoli or Ivanovic.
I don't think Sukova is overrated at all. I think she does deserve a place in the HoF. And I think she was very unfortunate to play during the Chris-Martina-Steffi years, losing 4 major finals to the likes of that trio. And to make matters worse, she beat either Martina (3x) or Chris en route to her finals. And to boot, she reached major finals 9 years apart! Some players (Jaeger, Morozova) had one or two strong years when they reached a major final. Sukova got to finals in 1984, 1986, 1989, and 1993, demonstrating she was a threat for a while. Shriver also played during this time, and may have had a higher overall ranking. Apart from her debut appearance when she reached the 1978 Open final, she wasn't winning big matches in majors (yeah she beat Martina again in 1982, but lost to Hana and would have been killed by Evert had she gotten to another final). Sukova was able to score her big win, and follow up with necessary just to get to another major final. So props to Helena for 4 major final appearances, but bad luck not to have won one.