I started to watch Serenity the other day.. was so bored that I had to leave it unfinished. I hate when that happens, lmao.
But, on topic of good sci-fi movies, Donnie Darko was a great one. I admit it can be confusing, but reading the fictional novel about time-travel on the website plus some other interpretations of the movie was enlightening, gave more to the experience. There was some "science" in the science fiction, imo, it wasn't just an action-movie or a horror-movie, or a very boring movie. Many sci-fi movies that I've seen have falled into few of these categories.
Like for instance, Alien works as a movie, yes, but I see it more as a horror-film than a sci-fi film. Not that there's anything wrong with that though!
Terminator 2 on the other hand is very entertaining action-movie. Sci-fi is in the idea but I see the emphasis being more on the action-side.
Rise of the Planet of The Apes was also a great one.
I see many good elements in different films, but the whole movie leaves something to hope for or doesn't quite reach the potential it has on paper. I'm fascinated with the genre though, obviously, since I'm here.