Personally, I think Best is an absolute stud! That being said, he's yet to prove anything. I would start Bradshaw. (I am starting him over Best on my own team)
I like Best but I'm going to have to go with Crabtree. I see a break out game for Crabtree vs the Seahawks secondary. Don't be afraid of him not playing any of the preseason games because he didn't play any last year and did fine.
I would go with Best as well. Bradshaw- how many carries, and where those carries will be (not in the redzone maybe, with Big Brandon Jacobs for short yardage stuff?) is too much of an unknown for me, although I would hang on to him and see how it all plays out in the next couple weeks. I generally prefer an RB to WR in a flex. Last (as much as it hurts a Hawks fan to say) odds of the Niners getting ahead and the relying on the ground game (Gore anyone?) to control the clock/pace might limit Crabtree to much, escpecially in the second half.