Live Free or Die Hard
The original Die Hard trilogy is fun action-y goodness, despite a slightly lackluster second movie. By comparison to Live Free or Die Hard, Die Hard 2 is actually not that bad at all. This is a simple example of modern action movies doing so much with action that there isn't really any semblance of realism anymore. When you start launching cars at helicopters, maybe it is time to stop and think about what you're doing.
What other terrible movie sequels have you seen?