One of the easiest to work with is Excel. This is because there are a large number of examples that can be found online that are easy to read and easy to use. The only problem is that they can take some time to download so you may not be able to get your work breakdown structure template up and running immediately.
Using the example of a work breakdown structure template is a good way to get your data into the format that you need. Of course, if you want you can always look at Microsoft Word or Excel, but that will take a lot longer to download and you will be making mistakes in the process.