I always lean towards running backs for the flex. In this case I'd start Justin Forsett since he'll be the starter for Seattle.
12 team H2H league, standard scoring, no PPR.
QB: Cutler, Kitna
RB: All Day AP, CJ2K, F.Jax, R.Brown, S. Greene, Dixon
WR: G. Jennings, Roy Williams, M. Williams (sea)
TE: J. Tamme
K: Hartley, Carpenter