For what you need, List<string> is probably the most versatile. StringCollection was handy in 1.1, when (without generics) you needed specific classes for typed collections. Collection<string> is handy if you want to subclass it to provide validation etc.
Either way, in 3.5, LINQ will provide lots of additional methods for them – but without LINQ, List<T> has more features (Find, Sort, etc)