Who is the best racehorse in the United States?

That question should be answered on Saturday afternoon, when a stellar field contests the $500,000 Marlboro Cup at Belmont Park. And the answer is likely to be: Greinton.

Virtually every healthy horse with championship aspirations has been entered in the Marlboro, which will be telecast by CBS (WDVM-TV-9) from 5 to 6 p.m. The East's best 4-year-olds -- Track Barron, Carr de Naskra and Vanlandingham -- are all in the lineup, along with the 3-year-old Chief's Crown. They are being challenged by a strong contingent from California -- Greinton, Gate Dancer and Al Mamoon.

Greinton doesn't have much of a popular reputation yet, because he is a typical product of trainer Charlie Whittingham: a European import who has bloomed relatively late in life. Undistinguished in France, he came to California last fall, and he has proceeded to finish first or second in 12 straight races. Despite his European origins and pedigree, he has been more effective on the dirt than on the grass. He ran a mile at Hollywood Park in 1:32 3/5 (two ticks off the world record) and won the 1 1/4-mile Hollywood Gold Cup in 1:58 2/5.

Greinton never has raced in the East, but Whittingham said, "I don't think different tracks will bother him. This is a very versatile horse. I think the change of tracks may even help him. He's training very well here. We've got everything going for us."

Greinton's main rival is Track Barron, who could collect a $1 million bonus if he wins the Marlboro Cup and the Jockey Club Gold Cup. He became eligible for it by winning the first leg of New York's "fall championship series," the Woodward Stakes, in his last start. His four-length victory had more than monetary significance. It was the first genuine blockbuster performance by a colt who generally has been overrated through much of his career.

Track Barron rarely has run fast, and frequently has been the beneficiary of good racing luck, but in the Woodward he led all the way to run 1 1/8 miles in 1:46 3/5, soundly whipping a good field.

In the Marlboro, however, Track Barron faces two new obstacles. He must go an eighth of a mile farther than he did in the Woodward, and he has failed in both his previous tries at 1 1/4 miles. Moreover, Vanlandingham conceded him the early lead in their previous two meetings, but he is unlikely to do so on Saturday, judging from his :33-flat three-furlong workout this morning.

"It might have been a tad faster than I wanted," trainer Shug McGaughey said. "But my horse is as good as he's ever been and I'm really excited about the race."

Another colt who has trained spectacularly for the Marlboro is Gate Dancer, who was disqualified in both the Kentucky Derby and the Breeders' Cup last year. He was unable to beat Greinton in three tries this winter, but he worked a spectacular mile in 1:35 3/5 here this week.

The Marlboro is only one of three significant stakes races on the Belmont card. An evenly matched field of six 2-year-old fillies will contest the $110,500 Astarita Stakes. Seven 2-year-old colts will compete in the 96th running of the Futurity Stakes, and Ogygian is expected to confirm that he is the best horse of his age in the country.