Coal is one of the best fuel sources that doesn't burn quickly and you can find it in several areas, being a balanced, cheap resource with acceptable quality. Higher-quality combustible resources will burn even more slowly inside your crafting stations, but you'll get a whole stack of Coal more simply instead. Dry Wood is a top choice as an alternative to any other fuel - as long as you chop plenty of trees to collect Wood or Branches - you can put these in a Dryer to get Dry Wood instead of looking for tree trunks that drop it, but the transformation of these resources into Dry Wood is slow. Oil is as good as Dry Wood, but using it just for burning can be somewhat troublesome when you need it for important items during your gameplay and struggle to produce it on a large scale, even though the process is not tricky. The Molten Heart is the slowest burner, but it's hard to obtain, taking it off the list as the number one choice.