You'd be surprised to know that Dave Roberts trails only Manny Ramirez, Barry Bonds, and Andre Ethier for MLB LFers in RC/G. Yep, he's been better than Carlos Lee, Alfonso Soriano, Carl Crawford, Adam Dunn, Reed Johnson, Juan Rivera, Raul Ibanez, etc.
Hafner is no longer underrated guys. He's getting a ton of publicity with his grand slams and the Sportscenter guys have started calling him Pronk. Most underrated should go to Dye from this list, but there are many guys that could fit the description, including Carl Crawford and Ibanez.