In Football it's clearly Barry Sanders, yeah they did have some success but nothing consistent as Barry was, also Dan Marino and Bo Jackson had it pretty bad but most of the time they had winning seasons, Marino only had ONE losing season I think.
In Baseball I would go with Tony Gwynn or Ken Griffey Jr.
In Basketball I always felt bad for Pat Ewing, those teams weren't terrible and were playoff regulars but they gooned their way into wins Ewing was the only real talent on that team. Rock, KG and Bron eventually got their rings, Ewing never did and that's partly due to his supporting cast.
I voted for Barry though because he basically retired because he knew the Lions were never going to get better and there was no way in hell they were going to let him go.