There is a hierarchy here:
interface IList<T> : ICollection<T> { }
interface ICollection<T> : IEnumerable<T> { }
You want to aim for the least possible coupling, so return an IEnumerable<T> if that is enough. It probably will be.
Return an IList<T> if the situation requires that the caller gets a List that it can use to Add/Insert/Remove. But even then it might be better if the caller created his own List from the IEnumerable collection.