6. Jerry Jones
Jerry Jones started off as one of the most successful owners in sports history. His Cowboys won three Super Bowls in his first six years as an owner. But those great Cowboys teams of the early and mid-90s disintegrated amidst a storm of egos, none bigger than his own, and now have not won a playoff game since 1997. Early in his time in Dallas, Jones clashed with another ego, that of his chosen first head coach, Jimmy Johnson.
Jones and Johnson's problems started with simple things, like Jones bringing celebrities onto sidelines and into the locker room, an example of his "this is my team, I can do what I want" attitude. Jones would often take credit for personnel moves that were not his idea and even at one point told reporters that he could coach the Cowboys.
Jones and his ego wore on Johnson (and vice versa) until the owner took Rick Gosselin and Ed Werder aside at a bar and told them it was time to fire Johnson (after consecutive Super Bowl wins) and bring in Barry Switzer, pointing out that 500 people could have coached the team to a Super Bowl (himself included). Eventually Jones did just what he said he would, firing Jimmy Johnson and hiring Switzer.
With Johnson gone, Jones had full control over personnel decisions, and despite one Super Bowl under Switzer (won mostly with talent accumulated under Johnson's regime) the Cowboys have not been close to the same with Jones refusing to give up control to a general manager or coach (the only coach he let have a large say on personnel after Johnson was Bill Parcells, and no surprise, Parcells is no longer in Dallas).
Jones may have helped to build a great NFL dynasty, but there is no doubt that his ego destroyed it.