Selecting wood decking for stairs
Durability.
Appearance.

The end. :)
I suspect that you're asking what to use in your particular case. That looks like painted pressure-treated SPF (spruce/pine/fir) in 5/4x6 (five quarter by six inches--about 1&quot; by 5-1/2&quot; actual). It's very common. Be sure to let it dry adequately before painting.
Some brands call that &quot;solid color stain&quot;, but to me anything that doesn't penetrate the wood and peels like that is just glorified paint. Anyway, look for that type of product to match what you have. If you're replacing it all consider the pre-stained &quot;cedar tone&quot; variety.