Which tree type yields the most wood?

Solution 1:

If you build tree farms underground plain trees are the best. If you keep the ceiling 7 blocks high the whole tree will be harvestable from the ground. Plain trees can grow when planted right next to each other unlike birch trees. Pine trees will not grow with a 7 block high ceiling.

However, if you plant plain trees too close together they won't grow much leaves. So it is easy to run out of saplings. I suggest planting them in rows and leave a block or two between the rows.

Solution 2:

In my experience, birch trees are the most practical for wood farms.

While they aren't as tall as pine or plain trees; they have the very desirable property of always being harvestable without building; making harvesting fast, and avoiding floating branches.

If you don't mind building a bit for getting at the wood, pine trees are probably a better choice, but they are less predictable.

Normal trees should be avoided at all costs, at least if you value your time and/or sanity.

Solution 3:

In my experience, pine trees are great. They are the tallest, and thus give the most wood.

@WillihamTotland mentions you have to 'build' to get all the wood - I guess he probably meant that pines are so tall, you can't chop the wood directly from the ground.
I always circumvented this issue by planting the trees next to one (or two stacked) blocks of dirt. This way you only have to climb on them and harvest all the wood. 2 blocks were sufficient in all cases.
You don't have any hassle with building the steps each time you chop wood and you get far more wood blocks than you would get from the same number of different type of trees.