Not many British horses on the lists!
Top of my flat racing list would be
Nearco (1935–1957) was an Italian bred Thoroughbred racehorse described by Thoroughbred Heritage as "one of the greatest racehorses of the Twentieth Century" and "one of the most important sires of the century." He was not only unbeaten, winning 14 races at distances from 5 furlongs (1,000m) to 1 mile 7 furlongs (3,000m), but also the patriarch of the most dominant sire line in Thoroughbred history.
Nijinski A beautiful horse to watch racing. Won the UK Triple Crown
Frankel, Won all his races with other jockeys saying "All you see is his arse!"
Black Caviar A phenomenal mare who doesn't know when she is beaten.
Native Dancer Sired so many good UK racehorses.
Nashwan Won the Epsom Derby and a lovely character.
Saddle Wells Another top racehorse and a leading sire.
All horses that not only proved their worth on the track but also at stud (except Black Caviar)
National Hunt racing is my favourite and my leaders would be
Red Rum - not only did he win the Grand National three times he was also second twice.
Arkle A fantastic horse who loved being the star. He was disappointing in his last race when he came second but it was found that he had broken his pedal bone. Gutsy horse.
Dawn Run Won not the Champion Hurdle but went on to win Cheltenham Gold Cup. She died on a race course in France.
Dessert Orchid A brilliant horse, won most of the top races.
Best Man Another brilliant jumper and a joy to watch. Won the cheltenham Gold Cup three times in the trot.
There are many others that I loved to watch, most jumped from fence to fence loving every minute of it.