Think about the pressure James Cameron had to deal with. Scott's 1972 original is praised as a groundbreaking entry in the sci-fi genre. Alien is a classic, but Aliens matches Alien in quality. Hell, 'till this day, Alien fans still argue over which film is better. More thrilling action, more extravagant set pieces, and as Dowds already mentioned, Aliens provides one of the most quotable lines in the history of cinema. Plus, Sigourney Weaver became the prime example of a tough, ass-kicking female hero, earning an Oscar nomination for Best Actress.
Forrest Gump isn't a bad movie, but Tom Hanks has delivered better performances in other films. Cast Away, Philadelphia, Road To Perdition, and The Terminal are a few examples. And after watch the TV spots and trailers, you might be able to add Cloud Atlas to the list at some point.
Movie fans have a soft spot for Hank's character. Forrest Gump is slow, but he has an endearing and charming personality, and his junkie/hippie friend (Jenny) constantly abandons him throughout the film, so it's easy to feel sympathy for him. But in the end, Forrest Gump is a long and drawn out comedy/drama. Along with Hanks, Gary Sinise delivers the second noteworthy performance, but is the rest of the cast really worth mentioning?
Gump has coasted off of the "it's okay to be stupid, as long as you try" message for years, but Aliens is remembered as an action/sci-fi classic.