It's an interesting question, though, worth some thought, and a quick look back through the records to remind even this old head of the glories of the past. In truth though, it is almost certainly entirely just to discount the achievements of virtually all of the fillies and mares that raced before World War II. American racehorses were quite simply better after the importation of European stallions and mares reached critical mass in the aftermath of the war. It seems pretty clear that the American racehorse reached something of an apotheosis in the 1960s and '70s, and everything since then must be compared to those horses. The fact that those decades happen to coincide with this old curmudgeon's impressionable teens and 20s has absolutely nothing to do with it.
So what does your list look like? Here's mine, with some semi-credible attempts at justification.
1. Ruffian....Ran too fast too many times not to be at or near the top.
2. Zenyatta....19 for 19....What else is there to say? (Yeah I know, there are plenty who quibble)
3. Dark Mirage...800 pound monster won 10 straight @3 and 4, many by huge margins
4. Gallant Bloom....Beat Shuvee 5 out of 6, 11 straight wins, vastly underrated even in her era
5. Allez France...Raced only once in U.S. when past her best, but beat Dahlia 5 for 5, beat colts 8 times in Europe
6. Personal Ensign...13 for 13, but had to be carefully handled because of soundness and thus difficult to evaluate fairly
7. Dahlia....only 15 for 48, but raced on when well past her best...Beat top colts silly in Europe 5 times, and 4 times in America.
8. Desert Vixen....9 for 11 at 3 in '73...admittedly a sentimental favorite. Led Dahlia a merry chase in D.C. International over distance way too far and almost held on. Great filly.
9. Rachel Alexandra....Sorry folks, generally overrated. Beat a bad bunch of colts, just like any of the fillies rated above her here would have.
10. Tosmah...17 for 22 at 2 and 3, beat colts in Arlington Classic
11. (why stop at 10?) Go for Wand...Another sentimental choice, but 10 for 12 before dying on the lead is pretty damned good.
12. Shuvee...Couldn't beat Gallant Bloom, but beat everything else after GB retired.
13. Bayakoa...16 for 21 and absolutely lethal at 5 and 6 when at her best.
That's my lucky 13. Have at it.