I think the old answer would have been Wars in America. However, the engine has been updated and changed quite a bit since then. For a good dose of gameplay using the AGE engine, the best would probably be Alea Jacta Est. It offers simple scenarios, a simplified recruitment panel, and the newer features like event cards and army/corps construction. Games like End all Wars and Civil War II are great, but they are challenging, have a larger scope, and are very time intensive.