You should avoid from returning List from a public API. 

The reasons are 1. Verisoning - if you'd have to change the implementation, you don't want to break your API consumers (e.g. You might find a need to override Add method which is not defined as virtual on List). 2. Simplicity - List is a bloated object and probably your consumers won't need all that functionality so it's better serving them something simpler.

You should retrun IEnumerable if all you need is to iterate over the collection / if you'd like to make it read only.

If you also want to allow operations like Add then return ICollection.

If order is important and you want to allow access to a specific index in the collection return IList (because this isn't possible with ICollection).