modo is super easy to use and extremely powerful. It’s also a lot more than just sculpting - it’s a poly/subdiv modeler, sculptor, renderer and animation package.
Zbrush is probably the most powerful sculptor on the market now. It’s also pretty difficult to use (IMO), and not as intuitive as modo.
Mudbox is pretty exclusively a sculptor. It’s probably more powerful with sculpting than modo, but doesn’t have the same modeling capabilities (it’s pretty much only a sculptor), and is less powerful than zBrush.
Check out Sculptris. It’s free, put out by the same people who make Zbrush, and while it lacks features it’s a really good sculptor. Probably would be fine for your needs and learning the basics.