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.
