Difficult Flex Options

I’m in the semifinals going against the 11-2 #1 seed. My team is as follows (standard league):
QB: Case Keenum
RB: Devontae Freeman
RB: Dion Lewis
WR: Michael Thomas
WR: Dez Bryant
TE: Gronk
FLEX: ???
DEF: Broncos
K: Will Lutz

I can’t decide my flex between these options:
Steffon Diggs
Jerrick McKinnon
Mike Davis
C.J. Anderson

Please help!