I just bought Predator and AvP1 on Bluray at Walmart for $25 total. The Alien 4-film set comes out on BD 10/26, so mark your calendars!
Anyway, it's hard to recommend not knowing what you already have. As mentioned, the Alien and Predator franchises have made some great flicks. Same with Terminator, Back to the Future, Transformers, Men in Black, Star Trek, Star Wars, Indiana Jones, Jurassic Park, Lord of the Rings, Pirates of the Carribbean, Spiderman, Batman, Tom Clancy films, Riddick, Matrix, and the Roland Emmerich films (Independence Day, Stargate, et al).
As the owner of several rooms' worth of DVDs and BDs, I can't even begin to think of a single one without thinking of 5 others.