I would wait until the end of the year to hand out the award for worst free agent signings. We aren't even at the All-Star break.
Renteria could turn his season around (not so sure about Taveras). Renteria's line drive rate is the same it's been his whole career, but his BABIP is down around 30 points. Not saying he will, but one could argue that he is due for a positive correction.
Milton Bradley's line drive is way down from last year, and his GB rate is up. I could see him ending up with worse numbers than Renteria.
Still, doesn't take much research to conclude that Taveras signing was not the best move.