Chris Young, 6 years, 31 million dollars
This contract was a rare one in which it was signed soon after Chris Young's rookie season. That doesn't mean it wasn't worth it though. He stayed with the team until after the 2012 season when he was dealt to the Oakland Athletics in a 3-team trade that brought closer Heath Bell to Arizona. Despite that, his time from 2008 to 2012 was still plenty worth the contract.
Chris had one All-Star appearance and a Home Run Derby appearance as well. From 2008-12', he played in 707 games, had 606 hits, 165 doubles, 98 HRs, 330 RBIs, 83 stolen bases, and 311 walks. Young struggled with making contact, but he had a great eye, power, and speed which enabled him to have two 20/20 seasons.
He hit .239, .323, .431, 97 OPS+, and garnered 13.4 WAR. That amount of WAR was easily worth the money as he would've gotten way more nowadays. Finally, he helped the team to two division crowns. Can't put a price on helping a team get to the playoffs.
Honorable mentions: Dan Haren, Justin Upton, Mark Grace, Aaron Hill, Zack Greinke, JJ Putz, Fernando Rodney, and Byung Hyun Kim.