Miike, replace it with the same thing that came off. The problem is probably not the wood but how the water is getting there to make the wood rot. Resetting your gutters and putting up screens to keep leaves and other debris out is key. Depending on the amount of rot, you may not even want to replace the boards. You can fill the holes with Bondo and it will last for many years if you first use a motor driven wire brush followed by a rotary tool (Dremel) with a small, coarse sanding drum to get out ALL of the current rot.