None is best. If you set poison out for the rats even if it is away from where your chickens can get to it the rats will take the poison and potentialy hide it (we used to poison mice in the house and every once in a while we would find a stash in a closet or somewhere) where that the chickens can get to it. I would suggest a snapping trap somewhere your chickens can't get to.
The best rat poison is NO poison. Bad idea. A rat terrier perhaps? One rat terrier holds the record for killing thousands of rats in just a few minutes. Maybe find a trap or two? I would refrain from putting any poisonous chemicals down where any other animal can get to it.
Unfortunately, the rats (or mice) CAN move it to where they can get it.
We put Decon out at our cabin, and in the spring came upon stashes far from where we put the bait (in the under counter cupboards) - under the couch cushion, piled in the corner on the kitchen counter, and even under the pillows on the bed. We've stopped using bait because we don't want our child to stumble across a moved bait pile and have her decide to try out the tasty green pellets.
Also, no rats here yet, but we have moles and mice, and I've caught my chickens a few times....ummm..."playing" with a dead mouse they killed, and a little while later, the mouse is no more. Didn't realize chickens make good mousers