WETA Digital in NZ did the computer animation for both Lord of the Rings and Avatar.
I suppose Lord of the Rings falls into the "Sci Fi/Fantasy" genre, but if you start to include fantasy the classification becomes unmanageable. You would have to include every movie involving dragons, movies like Jason and the Argonauts, Conan the Barbarian, The Golden Compass, Grindhouse, Jumanji and even The Polar Express. I don't view any of those films as real "Science Fiction."
As for Star Wars, I thought the series declined significantly after The Empire Strikes Back.
For me, "Science Fiction" involves the following elements: science (or at least pseudo science), space travel and alien species.
I think you also have to distinguish all of the movies based on a "post apocalyptic" future as a "sub genre" of science fiction. I'm talking about movies like Mad Max (and its progeny) the Terminator (and its progeny), Logan's Run, This Quiet Earth (nice little speeper), all of the Planet of the Apes flicks, Waterworld, the Postman, The Book of Eli, Children of Men, etc.
Then there is the "monster movie" sub genre: The Thing, Them, Predator, King Kong, and the Jurassic Park series.
My list of very good (but not seminal) Sci fi flicks would include Forbidden Planet, The Day the Earth Stood Still (the original with Michael Rennie), Blade Runner (directors' cut), Alien, E.T., Close Encounters of the Third Kind, Silent Running, Pitch Black, The Wrath of Khan, Outland, Event Horizon, Serenity, The Abyss, The Matrix (only the first one)Supernova and Stargate (the movie). I guess I would also include the following TV series: the original Star Trek, Firefly, Babylon 5 and the new Battlestar Gallactica.