Juan Pierre just announced his retirement. Adam Dunn retired after the end of the 2014. They were pretty much polar opposites as players. So who was better? Check out Ben Lindbergh's piece at Grantland, which also serves as an excellent primer on WAR and how it is applied to evaluate and compare players.
It's interesting: When both active and at their best in the mid-2000s, the sabermetric community adored Dunn and pretty much despised Pierre. That's why the answer to "who was better" is kind of fun.