I had this exact dilemma. I went with McKissic, on a gut instinct that could just be indigestion (it's thanksgiving up here.) Collins and Gallman are in better situations, but McKissic is probably the best athlete. He certainly has more burst than Lacy or Rawls.
I was was looking for a dart throw to pan out ok ROS, I weighed athleticism over opportunity (which isn't always very smart), and I'm a Seahawks fan, so that influenced my decision.
Collins is probably the safest long term, he does have Mark Ingram comparables, West is not good and Buck Allen is unlikely to become a workhorse, so he could be a ROS low-end rb2. But he has two fumbles already and was cut by my team so I wrote him off.
Gallman is in a pretty good situation this week, that offense is starting to roll, and he's walking into a 50/50 share. I don't see a ton of upside, but he could be a weekly rb3.
Full disclaimer: I'm just a dummy that likes playing ff, I try to get as much info on things as I can, but I'm still wrong all the time. I'm basing my opinion on what other people have written, and pure speculation on my part
*edit 12 team ppr also.