Richardson is the higher risk, higher reward play
Jennings is the safer play, but not as high a ceiling
Royster is another high risk/reward guy, but Ive been screwed by shanahan RBs in the past, and I dont even draft em now. If he starts, could be a stud. Or, he might see 3 carries and watch Helu run for 150 and 2 scores. who knows. I wouldnt start him. In fact, if he does do well this week, I would use that and try to trade him.
Greene, well, a subpar RB on a crappy team. He is at best a bye week fill in at flex, imo