It depends if you are in a dynasty league or not. I think Mathews will be the best short term option, but I think in the long run, Spiller will be the best of the bunch. Bradford will be good, but it's going to be a painful season for him this year. I like Best a lot. He's going to get the ball a ton. Gerhart is going to be hard pressed to get on the field unless Peterson gets hurt. WR's very rarely do anything in their rookie year. So looking at it from a dynasty league prospective:
Spiller
Mathews
Best
Bradford
Hardesty
Bryant
Ben Tate (I know he's hurt, but we are talking dynasty league)
Golden tate
Thomas
Clausen