Don't know who to start in my flex!

Hey Guys, not sure who I should plug into my flex. Duke Johnson, Emmanuel Sanders, Sammy Watkins, or Dion Lewis

This is a .5 PPR league and my lineup so far is :
QB: Big Ben
WR. Thomas
WR: Funchess
RB: McCoy
RB: Darwka
TE: Rudolph
FLEX: ???

I say Duke. I don’t trust that Denver offense, Watkins so far has been unreliable, and Denver D is too good against RB’s. Duke is a safer PPR start imo.

Thanks ! I was leaning towards either Duke or Sanders, but I think Duke might be the smarter play here.