What is the best wood to burn?

The simplest definition of the best wood to burn is wood that provides the greatest calorific value of heat for the lowest cost. This is predominately dictated by the moisture content of the wood rather than the species type although

As you can see from the graph below (based on a cubic metre of timber) Hardwoods tend to have higher density and as such higher calorific values (the increased density also dictates that hardwood has a lower green moisture content and as therefore as a rule will season quicker).

As you can see Ash is the hardwood with the lowest green moisture content which is why it is favoured as a source of firewood despite having a slightly lower calorific value.

 *Matthews, R and Mackie E Forest mensuration- A handbook for practitioners