Real GOATs of different sports. IMHO. Feel free to name/add your own.

[Bayard]

9. Mohammed Ali, of course, boxing

Women's Heptathlon: Jackie Joyner-Kersey. Six Olympic medals, "Greatest Female Athlete of the 20th Century by Sports Illustrated." Also, a darn nice person. We had seats at the Olympic Track & Field trials in Indianapolis, and were cheering for her. She turned around and gave us the biggest grin, and a wave.

Women's Marathon: Joan Benoit-Samuelson, first Women's Olympic Marathon gold medalist. She's also set 2 Boston Marathon records, and is still running marathons today. I have her autograph from Boston.
Grete Waitz is a close second, winning Silver in the same Olympic Marathon. She won the NYC marathon 9 times. She passed away from cancer several years ago. The one year I ran the Peachtree 10K in Atlanta, I saw her behind for about 2 seconds.

Dressage: Isabell Werth. Twelve Olympic medals in 6 Olympics, 7 gold ones. Nine time World Champion at the World Equestrian Games.

And of course, Secretariat, Super Horse of All Time. He always had an extra gear.