Obviously any sports with judges will be the least credible. Olympic sports like gymnastics, figure skating, ect always will have controversy or possible fixing, as well as combat sports like boxing and MMA.
Other sports can be fixed, but it's more difficult to do so. Of the American sports I would say the NBA is the least credible due to how subjective many fouls can be and how much refs can play a part of the game, as well as the Donaghyscandal obv. Baseball had the steroid era of course which hurt it's credibility.