Bad as Dunn has been, I gotta go with Navarro.
I always thought that the signing of Jaime Navarro was JR's knee-jerk reaction to the Cubs' swooping in and signing Kevin Tapani from the Sox. As such, I think JR let his emotions get the best of him by overpaying AND going (for him) so long, as if to say "I'll show THEM how to sign a FA pitcher"
And a really bad choice it was, even though it eventually did lead to Jose Valentin, who was one of my favorites while with the Sox. I was there for his 3-HR game in Kansas City, too.