Even in her prime years, I would say Nav was her toughest opponent, even more than Graf. I would also agree that Nav was a tougher opponent for Graf than Seles, but in the recent Tennis mag interview, Graf said Seles was by far her toughest opponent, so who knows?
Sorry but I had to laugh out loud at your assertion even an aging Navratilova (which is what she clearly was by the early 90s, lets face it) was Seles's toughest opponent in her prime over Graf. Unless you meant speculation if Martina were in her prime, then I could see a valid argument. Otherwise unreal. There is no way at all an aging Navratilova was a tougher opponent for prime Seles then Graf was. Navratilova never beat Seles in a major match in the early 90s in 4 attempts, and she was just creamed in 2 of their major matches which she never did to Graf. That was with the benefit of all 4 of those matches being on a surface that favored Martina. Graf always atleast took Seles to 3 hard sets when they played a major match, and slammed her brutally at that same Wimbledon where Martina gave her their only really close match in a major meeting. Graf had to play 3 of her 4 major matches with Seles on surfaces that favored Seles to boot. Seles never once destroyed Graf in a match, and often destroyed an aging Navratilova in matches.
Seles toughest opponents in the early 90s would rank:
1. Graf
------huge gap------
2. Sabatini
3. Novotna (hard to tell though since they didnt play much)
4. Capriati
5. Navratilova
These are the meetings of Seles vs Navratilova and Graf from April 1990- April 1993:
vs Navratilova
1990 ITALIAN OPEN CLAY F M. SELES defeats M. NAVRATILOVA 6-1 6-1
1990 VS OF LOS ANGELES HARD F M. SELES defeats M. NAVRATILOVA 6-4 3-6 7-6(6)
1990 VS OF CALIFORNIA CARPET (I) F M. SELES defeats M. NAVRATILOVA 6-3 7-6(5)
1991 VS OF PALM SPRING HARD (O) F M. NAVRATILOVA defeats M. SELES 6-2 7-6(6)
1991 US OPEN HARD (O) F M. SELES defeats M. NAVRATILOVA 7-6(1) 6-1
1991 MILANO CARPET (I) F M. SELES defeats M. NAVRATILOVA 6-3 3-6 6-4
1991 VS OF CALIFORNIA CARPET (I) F M. NAVRATILOVA defeats M. SELES 6-3 3-6 6-3
1991 VS CHAMPIONSHIPS CARPET (I) F M. SELES defeats M. NAVRATILOVA 6-4 3-6 7-5 6-0
1992 WIMBLEDON GRASS (O) S M. SELES 6-2 6-7(3) 6-4
1992 VS OF LOS ANGELES HARD (O) F M. NAVRATILOVA 6-4 6-2
1992 OAKLAND CARPET (I) F M. SELES defeats M. NAVRATILOVA 6-3 6-4
1992 VS CHAMPIONSHIPS CARPET (I) F M. SELES defeats M. NAVRATILOVA 7-5 6-3 6-1
1993 VS OF CHICAGO CARPET (I) F M. SELES defeats M. NAVRATILOVA 3-6 6-2 6-1
1993 PARIS CARPET (I) F M. NAVRATILOVA defeats M. SELES 6-3 4-6 7-6(3)
vs Graf
1990 BERLIN-GERMAN OPEN CLAY (O) F M. SELES defeats S. GRAF 6-4 6-3
1990 FRENCH OPEN CLAY (O) F M. SELES defeats S. GRAF 7-6(6) 6-4
1991 US HARDCOURTS HARD (O) F S. GRAF defeats M. SELES 6-4 6-3
1991 CITIZEN CUP CLAY (O) F S. GRAF defeats M. SELES 7-5 6-7(4) 6-3
1992 FRENCH OPEN CLAY (O) F M. SELES defeats S. GRAF 6-2 3-6 10-8
1992 WIMBLEDON GRASS (O) F S. GRAF defeats M. SELES 6-2 6-1
1993 AUSTRALIAN OPEN HARD (O) F M. SELES defeats S. GRAF 4-6 6-3 6-2
Like I said no comparision. You would never see Seles beating Graf at Wimbledon like she did an aging Navratilova, nor beating Graf 6-1, 6-1 in the Rome final like she did an aging Navratilova, nor beating Graf so easily in the U.S Open and WTA championships finals when she couldnt even beat her nearly that easily at the French Open. You are also comparing 4-3 to 10-4. I doubt an aging Navratilova was even Seles's second toughest opponent of this period. Sabatini and even Capriati did better vs her in general and on average then Martina did, especialy Sabatini. That is not a slight on Martina, the women was in her mid 30s, to even be remotedly competitive with Graf or Seles was an amazing feat on her part. However she was certainly not Seles's toughest opponent by this point in time.
As for comments by one of them it is well known Martina N. and Seles are very close friends, and it just as well known neither are friendly with Graf, especialy Monica. So that explains her end of the comments.
As for Graf's comments, Navratilova was past her prime by 1988 when Graf entered her prime, so of course Martina would probably seem less difficult to her then Seles. When she was playing prime Martina in 85-86 she was a little kid who was still developing strength and maturity, so probably in her own mind thinks it wouldnt have been that difficult if she were a bit older, which doesnt mean it is true. It just natural to appreciate more fully how difficult someone you played when you were both in your primes is.
All 3 are great players, but I doubt either Graf or Seles are at all qualified to accurately guage whether each other or Martina would be their tougher opponent in their mutual primes. Neither ever played Martina in their primes to know or compare really, except for a kiddy version of Graf playing prime Martina for a bit.