ESPN host Mike Greenberg has a new book out today called Got Your Number – the Greatest Sports Legends and the Numbers They Own. He’s talking about uniform numbers, the best who wore the numbers one through 100 on the back of their jerseys.

It’s a fun topic sure to start many friendly debates, for example, Number 32. Among the candidates: Jim Brown, Magic Johnson, Shaquille O’Neal, Sandy Koufax, Karl Malone, Steve Carlton, O.J. Simpson, Bill Walton and more. Who ya got? I’ll take Magic over the long haul, Koufax for the snapshot.

Number 34 is a whole lot of trouble, too. Who’s the greatest? Nolan Ryan? Hakeem Olajuwon? Shaq (again)? Charles Barkley? Big Papi? Walter Payton? Let’s hear it for Earl Campbell. It might wind up being Giannis Antetokounmpo, though.

Greenberg said he had the most difficulty picking the greatest player to wear Number 21. He and his stat helper narrowed it down to Roberto Clemente, Deion Sanders and Tim Duncan. Their qualifications:

Roberto Clemente: 15 time All-Star, 3,000 hits, Baseball Hall of Fame.

Tim Duncan: five time NBA champion, “the Big Fundamental,” 19 years all with the San Antonio Spurs.

Deion Sanders: two-time Super Bowl champ, 1994 Defensive Player of the Year, also played big league baseball.

Wrong, wrong, wrong.

Clemente was a great right fielder, but there was a better one in the same era, a guy named Henry Aaron. Duncan was terrific, but the Spurs’ titles were a team effort. Sanders was an amazing defensive back, but just a slightly above average baseball player (.263 career batting average). Also, a bit of a jerk.

Of course it’s unfair – and slightly ridiculous – to compare athletes of different eras because sports are constantly changing and evolving. For example, who was the more dangerous home run hitter: Babe Ruth or Barry Bonds? When the Babe played, it was pre-Jackie Robinson and 1947 so he never faced African-American pitchers. He also didn’t have to deal with late-night flights across three time zones. The westernmost city in baseball was St. Louis.

Of course, Bonds had his advantages, better equipment, better facilities, better nutrition, better coaching during his youth and, well, the obvious.

So in their own ways, Ruth and Bonds are close, undoubtedly the greatest of their time, but hardly comparable.

The best way to measure greatness, and ultimately who’s the greatest, is how a player compared to his contemporaries.

For my money, and I don’t think there’s any question, the greatest athlete to wear Number 21, in any sport, any era, happens to be from Houston.

Roger Clemens.

I know, he wore Number 22 while he pitched for the Astros and Yankees, but he was Number 21 for the bulk of his career with the Red Sox and Blue Jays and that’s when he racked up most of his spectacular numbers.

You want to crunch some numbers? The Rocket won 354 games and lost only 184 for a career winning percentage of .658. He struck out 4,672 batters (most ever in the American League and third most all-time). His career earned run average was 3.14. He led all of baseball in wins four times and earned run average seven times. He is tied for the most strikeouts in a nine-inning game with 20, but he’s the only pitcher to do it twice.

Most important, how did Clemens compare to other performers during his era? He won seven (!) Cy Young Awards given to the best pitcher in his league. That’s the all-time record and it’s two more than anybody else (Randy Johnson).

Let’s dig deeper. Clemens won the Cy Young Award for four different teams. He won his first Cy Young in 1986. He also won the American League’s Most Valuable Player that year. He won back-to-back Cy Youngs … twice. He did it with the Red Sox in 1986-87 and with the Blue Jays in 1997-98, both times wearing Number 21. Twice he won the Triple Crown of pitching, leading the league in wins, earned run average and strikeouts. He won his first Cy Young at age 23, his last at age 41.

That’s the winning number – 21 – and Roger Clemens owns it.