None of them seem like top fantasy contributors at this point. The Patriots want to run the ball, spread the ball out across a bunch of receivers, and if they have a #1 target right now, it's Watson. I think Troy Brown is their best WR at this point, just by nature of his experience and ability to get open, but there's enough of a cap on how well he can do that I wouldn't want to risk starting him. I don't think Gabriel was brought in to be the savior here; I think he's a guy who might end up with 40 catches if he's lucky. And Caldwell is still working himself into the mix.
I'd look to a receiver from another team right now, unless you either have to drop a guy that's out for season, like a Greg Jones, and want to pick up a lottery ticket like Jackson, or need a WR specifically for next week, in which case I'd lean towards Brown. Otherwise, wait and see who the hot waiver wire pickups are next week - there's bound to be a few, and all will look better than the Pats receivers at this point.