Dante Bichette's defensive numbers are pretty brutal if I remember correctly. It pretty much negated any offensive value he had. I'm not sure if he's the worst ever, though.
"There has always been a saying in baseball that you can't make a hitter, but I think you can improve a hitter. More than you can improve a fielder. More mistakes are made hitting than in any other part of the game."
- Ted Williams
"I know I'm the world's worst fielder, but who gets paid for fielding? There isn't a great fielder in baseball getting the kind of dough I get paid for hitting."
- Dick Stuart