Rudisha most, it would rule out the greatest performance in which I have full confidence, since the early 80s. If St Patricks and Fr Colm and a man who seems as decent and proud as DR are involved in doping then I would lose faith in all East African running.
Farah next because I think he is clean and like Radcliffe shows with utter dedication then you can achieve greatness. I think the points saying that the competition he faces isn't what it was 10 years ago is key in his current dominance. From cycling there was an interview with either Bardet or Pinot who said they simply weren't prepared to do what Froome, Wiggins etc did in terms of training, diet and dedication and therefore probably wouldn't win the TDF. farah didn't before but he does now.
Then Kiprop because it would take out the clean WR and because those Monaco races actually felt like the summer of 85 again (although big question about Aouita in retrospect.) MD has never been that good since and puts the current Diamond League in perspective.
Then Kemboi because it would have least impact internationally but would be siesmic in East Africa and hopefully be the catalyst for some much needed change.