It depends what you want to do with the result. If you need to get the count of items or get random access to individual items, go with an IList.
If callers just want to iterate through the items then go with IEnumerable - but you should document whether or not the returned value will be evaluated lazily or not - many IEnumerable instances these days represent queries that will be executed when the collection is enumerated. To be on the safe side, if what you are returning won't be evaluated on demand, I'd go with IList.
