Canadian-born hurler Fergie Jenkins was truly one who got away.
Signed by the Phillies as an amateur free agent in 1962, Jenkins worked his way up through the system and made a good impression when he was called up for the first time in September of 1965. He made one appearance for the Phillies early in the 1966 season in relief, and then he was summarily traded to the Cubs along with two other players for established pitchers Bob Buhl and Larry Jackson.
Buhl stuck around for a season in Philadelphia before calling it quits at age 38, while Jackson was a useful starter for three years until he retired after the 1968 season. Jenkins, however, took off like a rocket. He would rattle off six straight 20-win seasons for the Cubs, highlighted by his NL Cy Young win in 1971. From there, Jenkins had two separate stints with the Texas Rangers sandwiched around two years with the Red Sox, and he then wrapped up his career with two more years with the Cubs.
All told, Jenkins won 284 games and struck out 3,192 hitters while posting a 3.34 career ERA in a shade over 4,500 innings pitched before landing in the Hall of Fame in 1991. And the Phillies didn't reap any of the rewards of his arm.
I suppose we should also give a passing mention to Ryne Sandberg here, although he now retains a Voldemort-like reputation among Phillies fans thanks to his abject failure as a manager. It was eerie how, 16 years after it happened with Ferguson Jenkins, the Phillies sent another young player to the Cubs only to see them end up with a statue at Wrigley Field and a plaque in Cooperstown. Let's be glad this trend seems to have ended.