White oak "heart" wood is rot resistant - white oak sap wood is not. Be sure that your wo boards don't have any sapwood in them.

Hemlock and black/yellow locust are both extremely rot resistant. Osage Orange is even better. Black Walnut heart wood is also rot resistant.

Ipe is about the best that you could use.

Pressure treated pine rated for ground contact will also be very durable.

Cypress, the various cedars are softer but also rot resistant.