It depends on what you're doing, as each format is designed to be (fairly) easy to use, though each is also designed for a certain audience or type of project.
Harlowe is generally easy to get started with, but it's hard to extend it's functionality. This can make it more user friendly initially, and I quite like it's syntax personally. But if you need features that aren't included, or if you need finer control over certain systems like the history system, it can be a pain.
SugarCube is also fairly easy to get started with, but has the added bonus of being easy to extend as well, and it gives the author a great deal of options for fine-tuning. SugarCube includes some features that might not make a lot of sense to new users, but these things can usually be ignored until you need them.
Snowman is mostly designed for web programmers or for people who are using Twine as a component of a larger work.
My personal recommendation is Harlowe for simpler works, and SugarCube for longer or more complex works.