The balloting for horse of the year has stimulated lively debate in racing circles this week. Since the Eclipse Awards were established, there have never been three candidates with credentials like Affirmed, Exceller and Seattle Slew.

The choice is especially difficult because no one has ever defined what a horse of the year is supposed to be. (The best horse at a mile and one quarter? The horse with most brilliant single performance? or the one who sustained his excellence through-out the season?) Given such latitude, I felt free to cast my vote on the basis of sheer personal prejudice.

Seattle Slew was the most gifted thoroughbred to race in America in 1978. He won two-thirds of Belmont Park's fall championship series, and his nose loss in the Jock Club God Cup was even more brilliant than his voctories. Only a champion could have battled head and head with Affirmed at a suicidal pace for six furlongs and still be able to fight back when Exceller came flying at him in the stretch.

Unfortunately, if Seattle Slew does win the voting, he will not trot onto the stage at the Fairmont Hotel in San Francisco Feb.9 and whinny gratefully as he receives the Eclipse Award. The trophy will go to his owners-Mickey Taylor, Jim Hill, and their wives-and they deserve no honors.

Rarely have owners have been so spoiled by success. Rarely have owners managed a good horse so badly, and yet so arrogantly.

It was Taylor and Hill who insisted on sending Slew to race in California last year over the objections of the trainer who had managed the colt flawlessly until then. After Slew lost, they fired the trainer.

When Slew suffered second defeat of his life at the Meadowlands this fall, jockey Jean Cruguet correctly pointed out that he had been undertrained. The owners promptly fired Cruguet.

Taylor and Hill mapped out Slew's 1978 campaign the way Woody Hayes plans Ohio State's football schedule. They wanted to face an absolute minimum of real challenges. They entered Slew in only two or three races in which he risked defeat. At the end of the season, when the colt could have proved is versatility by running on grass in the Washington D. C. International, he closed his earer running against nonetities at Acquaduct.

Such a philisophy is the bane of European racing. Good horses in England and France are often pointed for two or three major races a year, so they can establish their value, and then be whisked off to stud. Such tactics may be smart economics, but they are not bad for the sport, and they should not be rewarded with a horse-of-the-year title.

Affirmed and Exceller, in contrast, were managed the way race horse should be. They never ducked a challenge.

Affirmed provided the sport's most exciting moments of the year in his Triple Crown victories over Alydar, but Alydar, was the only first-class rival he defeated all year. Affirmed was soundly beaten by Slew in both their meetings this fall.

Exceller's whole season was exemplary, although he didn't get proper recognition because he spent much of it in California. He won seven of 10 starts (all major stakes), won on grass and dirt, proved himself as a good weight-carrier and earned $879,790. Most importantly, he beat good horses at the top of their game.

In the spring, Bowl Game and Noble Dancer II were the two best grass runners in the East. They were in razor-sharp form when they went to Hollywood Park for a meeting with Exceller. He blew them away.

In the summer, the best older horse in the nation appeared to be the California-based Vigors, who had won all four starts spectacularly. Exceller ran a mile and one quarter in 1:591/5 to beat him in the Hollywood Gold Cup.

In the fall, havin been in training all year, Exceller came East to face a fresh Seattle Slew, and split two decisions with him. He provided sufficent justification for casting a vote against the Slew Crew.