The best wood to burn in Minecraft is called wooden slabs. What is wooden slabs? Wooden slabs are known as a half block made entirely of any type of wood. They are decorative blocks that are able to make house or structures look better. All of them are able to be used as a longer alternative to stairs, as a player and mobs automatically walk on the top of half blocks without jumping.
Aside from that, wooden slabs are also able to be used as an alternative fences, as one is able to create a one-and-a-half hit-box similar to fences by placing a slab on the top of a regular block. Top based slabs are able to support blocks on their top side. Redstone located on the top of a slab cannot power a redstone wire diagonally downwards, but is able to be powered by that redstone wire. This one does not obstruct redstone currents in the way that the solid blocks do. On top of that, wooden slabs are also able to be used as fuel for furnaces, blast furnaces and smokers, smelting ¾ of an operation.
The main reason of why wooden slabs are named as the best and the most efficient fuel in Minecraft is because you are able to make a lot of them out of just a few logs. Basically, 3 logs make 12 planks, which in turn create 24 wooden slabs, which all burn for 15 seconds. On the other words, just 3 logs give you 360 seconds of burning time, or little over 1/3 of the burning time of a lava bucket.
Once again, wooden slabs are considered as the best wood to burn in Minecraft. To name which one is the best wood to burn, here is the comparison of the other woods.
Coal vs charcoal
It might look strange that the coal is less efficient compared to charcoal. Actually, the thing has to do with the time it takes to acquire the same amount of every item. In general, trees are way more widely available and renewable, while coal needs more effort and a bit of luck to find in large quantities. That aspect makes charcoal generally a much more efficient fuel compared to coal, as long as you keep replanting saplings.
Charcoal vs wooden planks
Charcoal is less efficient compared to wooden planks, based on burning time and farming time. Despite burning for 80 seconds, this one is made by first burning a log for 10 seconds. The same log could be turned into 4 planks, meaning it will burn for a total of 60 seconds, which 4 planks could smelt 6 items, as it takes 10 seconds to smelt 1 item. A lump of charcoal lasts 80 seconds, that smelts 8 items, but you need to add the 10 seconds it takes to burn a log to make the charcoal. It means 1 item takes 11.25 seconds to be cooked if you use charcoal, while wooden planks will only take 10 seconds per item.