For me it was between the 80s and 2000s. For me I picked the 80s though since while 2006-2009 has been the worst womens field in history 2000-2005 was actually reasonably good. For the 80s only 1980-1981 and 1989 had decent fields.
Why would 1989 be a decent year and 1982, 1984-1987 would not?
For me, 1988-1990 were transition years with the comings and goings of great players, some well past their prime and some on the verge of, but not yet hitting their stride.
In 1982, you still had Martina and Chris playing at a high level. Jaeger's game was maturing and becoming more aggressive. The only sore spot was that Tracy and Hana missed significant time with a back injury that would plague both for a while to come. But both were still effective. The ending to that year with the Australian and the Toyota Championships provided suspense as to who was really #1 with both Martina and Chris coming up with huge wins.
1984 and 1986 were probably Martina's two best years. She may have dominated them, both we give Graf a lot of credit for dominating in similar fashion as she did in 1989 without criticizing the field. Chris was still winning slams, Hana was competing at a high level in both years. In 1984, she couldn't beat Chris. And in 1986, she couldn't beat Martina. But she was beating everyone else. Sukova made an impact in 1984 and was one of the most consistent competitors of 1986. Steffi and Gaby were not mature yet, but already making considerable waves. Not unlike how Seles was in 1989.
1985 was another year that ended with drama. Chris and Martina traded French and Wimbledon titles. Hana upset the apple cart at the US Open by beating both. And the #1 ranking was again on the line at the Australian with Martina having to beat both Chris and Hana in 3 sets to win the title and the ranking.
And 1987 was a rare year at any period of time in womens tennis where you didn't know what was going to happen. Steffi dominated early. But Martina came through against her at Wimbledon and the US Open. Chris didn't win a slam, but played two great matches that year in Houston and at Wimbledon vs. Martina. Hana didn't have a comlpete year after she was injured, but still managed to beat Martina in Australia.
If I was to rank the best years of the 1980's, I would put 1989 somewhere towards the middle, and well behind years like 1980, 1981, 1982, and 1985. Those years would stand up well in any decade.