That's easy, Arian Foster straight up for Lawrence Maroney (this was right before Ben Tate was injured, who was thought to be the starter). As I recall the other owner had Tate and we were just trading handcuffs (Maroney was the backup somewhere, the team escapes me now).
The only good thing about it is that the owner I traded him to didn't win the championship that year (was eliminated in the first round of the playoffs) and the league folded the following year.
My worst by far was right before the beginning of the 2010 season:
I got: Sidney Rice and a future 1st Rounder
I gave: LeSean McCoy
It seemed like a no-brainer at the time. So much so, that another owner tried to veto the trade. Oh lord I wish he would have succeeded! Rice had just had his monster year, and due to his injury concerns I viewed him as a buy low. The groupthink of footballguys hurt me on this one. I was always really high on McCoy, that was part of the reason I ended up drafting him, but I started to question myself after an average rookie season, and seeing all the experts bashing McCoy. I should have stuck with my gut, and realized McCoy was something special.