For me I'd have to say Shin Soo Choo. At the time I wasn't very aware of his skill set or his potential. I generally wasn't paying much attention to our farm system at the time. As far as I knew, he was just another prospect that sucked every time we brought him up to the big leagues. Plus, we got Ben Broussard who was having a good year and was a proven major league hitter. UGH. Now all I can say is "What the fuck were we thinking?" An honorable mention goes to Carlos Guillen. I definitely didn't see that one coming. All he did was get hurt, then all of a sudden he became a pretty damn good major league hitter. So, who did you want gone and is now making you rue the day the Mariners traded him?