Who has Zenyatta beaten?

Summer Bird and jockey Kent Desormeaux win the 2009 Travers Stakes at Saratoga. AP Photo

Nineteen up, nineteen down. Zenyatta has done it all. Her quest for a pair of perfect 10s comes in a few weeks at Churchill Downs with the world watching in the Breeders' Cup Classic. But there is an undeniable question standing in her way to 20 consecutive victories -- the competition.

Even the most staunch supporters of Zenyatta, and put me at the fore as I think she'll win the Classic for the second straight year, must admit that this will be, by far, her toughest test. Forget the discussion about the dirt surface. The hurdles left for Zenyatta to climb are horses like Lookin At Lucky, Quality Road and Blame. Just how good are those three? To me, they are far and away the three best main-track horses Zenyatta will have raced against in her career.

See where you think they would rank among the competition she's faced in her career. I've ranked Zenyatta's 19 best opponents over her 19-race winning streak. As you'll see, there are a lot of horses she dispatched in her memorable 2009 Breeders' Cup Classic win. Female runners (fillies/mares) are denoted with the (f).

19. Switch (f): A rising 3-year-old filly that upended this year's front-runner for championship honors (Blind Luck) in the Hollywood Oaks and has potential to be one of the West Coast's leaders upon Zenyatta's retirement. She was nearly up to the task against Zenyatta when nabbed late in the Lady's Secret this month at Hollywood Park.

18. Girolamo: Now a Grade 1-winning sprinter of more than $400,000 and still active, he was thrown to the wolves in the 2009 Breeders' Cup Classic at a distance beyond his scope.

17. Regal Ransom: A still-active earner of $1.8 million, he won the UAE Derby and Super Derby at age 3, beating, among others, Blame. He was a pace casualty in the '09 Breeders' Cup Classic and is more suited at middle distances.

16. Tough Tiz's Sis (f): Grade 1 winner of the 2008 Lady's Secret Stakes on Santa Anita's synthetic surface and the 2008 Grade 1 Ruffian on Belmont's dirt track, she earned just over $900,000. Her Ruffian score came via the escape route after back-to-back losses to Zenyatta at Del Mar and Santa Anita.

15. Mine That Bird: While he's become the butt of jokes among hard-nosed horseplayers, lest we forget he was Canada's champion 2-year-old and finished in the money in the Kentucky Derby (win), Preakness (place) and Belmont Stakes (show) -- and gave Rachel Alexandra perhaps the biggest late threat of her banner 2009 season. Mine That Bird was a spent horse by the time he was no factor in the 2009 Breeders' Cup Classic. But he's earned $2.2 million and that's no joke.

14. St. Trinians (f): A heart-pounding runner-up to Zenyatta in the Grade 1 Vanity this summer at Hollywood, she is freakishly good on the synthetic tracks of California. While a winner of just over $300,000 lifetime, she's 7-for-13 and handled defending Breeders' Cup Ladies' Classic winner Life is Sweet in the Grade 2 Santa Maria Handicap this year. You could argue she doesn't belong ahead of some on this list, and I'd concede there's some projection involved here ranking her higher than her resume boasts.

13. Cocoa Beach (f): Third to Zenyatta in the 2008 Breeders' Cup Ladies' Classic and also third behind her in the 2009 Lady's Secret, this $1.7 million earner was best known stateside for winning the Grade 1 Beldame on dirt and Grade 1 Matriarch on turf.

12. Hystericalady (f): This 11-time winner of nearly $2.4 million was a photo-finish second to Ginger Punch in the 2007 Breeders' Cup Ladies' Classic at Monmouth on a sloppy dirt track and winner of the Grade 1 Humana Distaff at Churchill on dirt. In 2008, she chased Zenyatta home in the Lady's Secret and the Breeders' Cup Ladies' Classic to close her career.

11. Awesome Gem: A throw-back to another time, this 38-start veteran has earned $2.2 million with consistency over pizzazz. He finished third in the 2007 Breeders' Cup Classic at Monmouth on dirt, has won the Grade 1 Hollywood Gold Cup on synthetics and the Grade 2 Hawthorne Gold Cup on dirt. He was no match for Zenyatta in the '09 Breeders' Cup Classic.

10. Music Note (f): Runner-up to Zenyatta in the 2008 Breeders' Cup Ladies' Classic on synthetic, she was a commanding dirt winner of the Grade 1 Gazelle, Coaching Club American Oaks and Mother Goose Stakes in 2008. She added the Grade 1 Beldame and Grade 1 Ballerina in 2009 before finishing her career third in a second attempt at the Ladies' Classic behind Life Is Sweet.

9. Life Is Sweet (f): Once known just as Zenyatta's stablemate, she made a name for herself in winning the 2009 Breeders' Cup Ladies' Classic while her partner tallied the Classic the next day. Life Is Sweet chased Zenyatta in races like the Milady and Lady's Secret while earning $1.8 million-plus, but she also creditably ran third in the '09 Grade 1 Hollywood Gold Cup against the boys.

8. Richard's Kid: Sixth in the 2009 Breeders' Cup Classic, this deep-closing veteran has won the past two editions of the Grade 1 Pacific Classic, not to mention last month's Grade 1 Goodwood. Add in placings in the 2009 Grade 1 Goodwood and 2010 Grade 1 Hollywood Gold Cup, and this late-bloomer has rattled off $1.7 million. Lest we forget he was a stakes winner on dirt over Grade 1 winner Bullsbay before he was sent for a synthetics career revival.

7. Colonel John: Versatile earner of $1.7 million won the 2008 Travers on dirt, the 2008 Santa Anita Derby on synthetics and the 2009 Wickerr Stakes on turf. Colonel John finished fifth in the '09 Breeders' Cup Classic at age 4, his same placing as in the '08 Classic behind winner Raven's Pass.

6. Ginger Punch (f): Winner of the 2007 Breeders' Cup Ladies' Classic at Monmouth Park, this 12-time winner of more than $3 million was no match for Zenyatta in the 2008 Apple Blossom at Oaklawn. Beaten that day by eight lengths on natural dirt, some have said Zenyatta caught her on the downside of her career, but that's a mistake. Ginger Punch won her next four races, including a trio of New York-based Grade 1s, the Phipps, Go For Wand and Personal Ensign.

5. Rip Van Winkle: A tiring 10th in the 2009 Breeders' Cup Classic while bet to near co-favorite status with Zenyatta, he has rebounded this year to win such prestigious races as the Group 1 Juddmonte International, to add among his $1.9 million earned. Immediately preceding his '09 Classic meltdown at Santa Anita he had won the Group 1 Sussex and Group 1 Queen Elizabeth II Stakes and was beaten by a nose recently in his title defense of the latter.

4. Twice Over: An active earner of just under $3 million, he won the Group 1 Champion Stakes this month in Newmarket for the second straight year, this after winning this summer's Group 1 Eclipse at Sandown. Twice Over finished third to Zenyatta on the synthetic surface at Santa Anita in the 2009 Breeders' Cup Classic. While he's never raced on dirt, his international class is unmistakable.

3. Einstein: An 11-time winner on all surfaces of nearly $3 million, he rates as one of the more versatile and consistent horses of the past decade. Though well-beaten with traffic issues when 11th to Zenyatta in the 2009 Breeders' Cup Classic, his career was full of highs. In 2009, he won the Grade 1 Santa Anita Handicap on synthetics and added his second score in the Woodford Reserve Turf Classic at Churchill. On dirt, he won the 2008 Grade 2 Clark Handicap and was second to Curlin in the 2008 Grade 1 Stephen Foster, while running third in the Grade 1 Donn Handicap.

2. Summer Bird: The champion 3-year-old male of 2009, he won the Grade 1 Belmont Stakes and Grade 1 Jockey Club Gold Cup on Belmont's traditional dirt track, as well as the Grade 1 Travers on dirt at Saratoga. Running fourth, beaten by three lengths by Zenyatta, the 2009 Breeders' Cup Classic would be his final career start. While he made just nine starts lifetime, Summer Bird was a distant second to Rachel Alexandra in the Haskell Invitational and third in the Arkansas Derby. He earned $2.3 million in one eight-month season.

1. Gio Ponti: Runner-up by a length to Zenyatta in the 2009 Breeders' Cup Classic, this active star is approaching $5 million in career earnings with 11 wins on his resume. Gio Ponti is the winner of such prestigious Grade 1 turf races as the Arlington Million, Manhattan and two editions of the Man o' War. Recently he added the Grade 1 Shadwell Turf Mile to his resume in preparation for a return to the Breeders' Cup World Championships. Will he compete in the Classic again, or stick to the Mile on turf?

Not only would I affirmatively rank Lookin At Lucky, Blame and Quality Road one-two-three in order of Zenyatta's strongest competitors ever, it's alarming to notice that none of the 13 best to challenge Zenyatta in her career were horses she has faced in the current year, 2010. There's a certain amount of ducking on both sides of the aisle responsible for that as she's raced exclusively in Grade 1 races this year for fillies and mares.

But it's a fair question to wonder if Zenyatta is as good as the horse we saw in 2009, given the lack of any serious opponent in 2010. If the Racing Gods cooperate, we'll soon find out, once and for all time.

Jeremy Plonk has been an ESPN.com contributor since 2000 and is the owner of the handicapping-based website Horseplayernow.com. You can e-mail him at Jeremy@Horseplayernow.com.