What is the  best mare or filly of all time?

Mo, the little Polish Arabian mare I had for my kids.

She was fast and quick enough for barrel racing, happy to jump, happy to move forward, and just as happy to stop when requested.

She could be trusted even with the four year olds.  Once I had a nephew on her back, and he was supposed to be walking.  He saw his older brothers trotting on the other side of the field, and decided he could do that too!  So he turned her sharply towards them, clapped his heels on her sides, and she took off like a shot.  I yelled  "Mo!  Whoa!!" and she stopped dead.

All safe, all sound.

A horse like that - kid safe, but spirited and talented, is one in a billion.

Best in what way exactly?