Rating the top prospects for the Kentucky Derby is difficult at this time of year because the contenders are so far-flung. With good 3-year-olds in Florida, New York, Arkansas and California, and sometimes in Maryland and Louisiana, even the most conscientious handicapper would find it hard to know who is better than whom.

To overcome these geographical barriers, I joined forces with two other journalist-handicappers, Randy Moss of the Arkansas Democrat and Jay Privman of the Star-News in Pasadena, Calif. The three of us calculate speed figures in the same way, so we can compare the 3-year-olds with some precision. When Privman reported that Image of Greatness earned a figure of 99 while winning the West's most important 3-year-old race to date, I knew immediately that there are a dozen colts in Florida who are superior to the best in California.

Nor is there a single horse in Arkansas or Louisiana who can be considered a legitimate Derby contender, despite some provincial sentiment for such colts as Clever Allemont, Tiffany Ice and Northern Bid. In fact, there are so few legitimate 3-year-olds in America that a top-10 list for the Run for the Roses only reaches to seven.

1. RHOMAN RULE. From this generation of 3-year-olds I have seen only two performances that have excited me. One was Rhoman Rule's third-place finish in the Laurel Futurity last fall. The other was his 10-length victory in his season debut at Gulfstream Park. He subsequently won the Everglades Stakes at Hialeah by eight lengths.

He fits the profile of a Derby horse neatly. His trainer, Angel Penna Jr., has experience handling good horses. His jockey, Jacinto Vasquez, has won the Kentucky Derby twice. He has a pedigree that suggests he can go 1 1/4 miles. Rhoman Rule might next run in the March 30 Flamingo Stakes, but he is more likely to wait for the Wood Memorial at Aqueduct on April 20. In either spot he should prove what I already believe strongly: that he is the best 3-year-old in America.

2. CHIEF'S CROWN. The Eclipse Award winner of 1984 has done very little wrong in his career; he is consistent and versatile, a professional athlete. Yet he is hardly brilliant; he has never run a race as fast as either of Rhoman Rule's last two victories. Chief's Crown has not trained impressively this winter, and his victorious debut in an allowance race was hardly awe-inspiring. He must be taken seriously as a Derby contender until he flops, but even though he is the reigning champion of his generation he has a lot to prove.

3. ROO ART. Maryland racing fans know what the rest of the world does not: that this Laurel-based colt is blessed with great talent. A powerful stretch-runner, Roo Art won four races in a row before losing a minor race at Aqueduct that was surely just a tuneup for more important objectives. Roo Art's trainer, Barclay Tagg, has never competed in this league before, and that is always a factor in Derby handicapping. But Roo Art belongs in the big leagues; from the speed-handicapping standpoint, he is every bit as good as Chief's Crown and Proud Truth, the nominal leaders of this generation.

4. PROUD TRUTH. He has won five of six career starts, and Las Vegas lists him as the 5-to-2 future-book favorite to win the Kentucky Derby, but Proud Truth doesn't impress me much. His performances have been consistenty slow. Moreover, he will have campaigned steadily from December through the spring, and he might be past his peak by the time he gets to Churchill Downs.

5. STEPHAN'S ODYSSEY. He lost twice to Proud Truth this winter, but is a lightly raced colt who probably has not yet shown how good he may be. With the great Woody Stephens training him, Stephan's Odyssey will be primed to deliver his optimum performance on Derby Day.

6. NORDANCE. When Nordance won an allowance race at Aqueduct earlier this month, speed handicappers were left agog. This was not merely the fastest race run by a member of his generation; it was a performance that would have been worthy of Seattle Slew or Affirmed.

It was probably somewhat fluky, but Nordance did come back to win a minor stakes race in which he dealt Roo Art the first loss of his career. Nordance still looks like a one-dimensional front-runner who wouldn't be capable of winning a race like the Derby, but his trainer Allen Jerkens has made a career of working miracles.

7. IRISH SUR. He has already been beaten convincingly by Proud Truth and Rhoman Rule, but in such a thin crop of 3-year-olds Irish Sur still must be considered a marginal contender. At least he has the stamina and stretch-running style to win the Derby.

That's the whole list; I can't make a legitimate case for any other 3-year-old in America. I would happily take even money that the name of the Kentucky Derby winner is on this list. And I have already taken much juicier odds that the horse on the top of the list, Rhoman Rule, will be draped with roses on the first Saturday in May.