I've never been a huge Star Wars fan. However, in my opinion, The Empire Strikes Back is by far the best of the Star Wars films. To me, it had the stronger story, easily the most character development of the leading characters, the characters were more endearing overall and the revelation of Vader being Luke Skywalker's father had to be a major shock to moviegoers in those days. The action didn't have as big or an epic feel to it as the first or the third films, but the story that was told and finally learning some of actually what a Jedi is and is all about more than made up for that.
American Beauty, to me, is a great drama that featured a great cast that all performed their roles to near perfection. Kevin Spacey & Annette Benning, for instance, were the perfect example an unhappily married, middle age couple desperately looking for something meaningful in their lives. You both liked and disliked either of them because you could see both their good and bad qualities.
In the end, I'm going with Star Wars, however, because it's just simply the more memorable and influential movie. American Beauty, while having a great cast, was ultimately just another drama with a few laughs tossed in here and there. The Empire Strikes Back is one of those movies that, 30+ years later, people still talk about and compare.