Summary: The most popular collection class -- be careful in perf critical situations

ArrayList

""In Visual Studio 2005 consider using List<T> anywhere you might have otherwise used ArrayList as it offers superior performance in almost every situation. In particular foreach (...) over an ArrayList is much slower than the equivalent operation over List<T>. In performance critical situations consider using for (...) instead of foreach (...) when iterating over an ArrayList.""

See PerfSystemCollectionsGenericList for additional information and caveats.