Who was the best mare to compete vs. the boys?

[jupiter]

Tarport Hap on the pace, Moni Maker and Roquepine on the trot.

[SDST2009]

I lean much more toward the trotting side to will weigh in on that gait only:

Moni Maker
Une De Mai (came here to say this and was pleased to see others had beaten me to it)
Delmonica
Fresh Yankee
Peace Corps

More recently..Hannelo re, Atlanta, Peaceful Way and Manchego impressed.

That said, there are a good many mares who deserve honorable mention for dominating their local circuits at the open level.

I maintain the belief that if the talent level is the same/comparable, a mare will nearly always out-game a male competitor, horse or gelding.

[ryansdad]

   Grade Singing Tarport Hap Manchengo

[Meadow Ford]

How about the trotting mare that did not get raced till she was eight years old?
Won her first lifetime starts at Goshen in 2:36 and 2:37.
Won 350 heats lifetime.
Won 95 of the 123 races she was entered in.
Several heats per race.

Several decades ago I told an owner to pick up his mare because she was not worth wasting his money and my time on. He picked her up and paid his training bill.
He then wrote me a nasty letter writing that I did not have the patience to wait on a mare not even a mare like Goldsmith Maid and get her to the races.
He was 100% correct!