The best natural wood for greenhouses depends on several factors, such as climate, budget, and personal preferences. However, there are several wood species that are commonly used for building greenhouses, as due to their natural resistance to decay and insects, as well as their durability and appearance.
Cedar: Cedar is a popular choice for greenhouses because of its natural resistance to decay and insects. It is also lightweight, easy to work with, and has a beautiful natural color.
Redwood: Redwood is another durable and insect-resistant option for greenhouses. It has a beautiful color and is naturally rot-resistant, making it a good choice for humid or wet environments.
Cypress: Cypress is a strong and durable wood that is resistant to rot and insects. It is also more affordable than cedar or redwood and is a sustainable choice.
Douglas fir: Douglas fir is a strong and durable softwood that is often used for outdoor construction. It is more affordable than some of the other options and has a natural resistance to decay and insects.

Ultimately, the best natural wood for your greenhouse will depend on your individual needs and preferences. It is important to choose a wood species that is resistant to decay and insects, as well as one that can withstand the climate in your area.

Also, Please make sure to choose wood that is sustainably sourced and harvested to reduce the environmental impact of your greenhouse construction project.