I have long stated that Aliens is my favorite movie of all time. My criteria for determining that is that I can watch the bloody thing over and over and over again and NEVER get tired of it.
I just watched Serenity for the second time, and after having watched it, I went back and revised my 4 1/2 star rating to a solid . Why? As a sci-fi/action flick, the thing is almost without fault. I've seen all the episodes of Firefly, but even if I hadn't, I think the movie would still have knocked my out of my seat.
Like Aliens, Serenity has a killer script and moves along with a tight pace that never leaves you bored and looking at your watch. Both films have a very specific personality that just fits, and characters that are well fleshed out, real and that you come to care about within the scope of the film.
Having watched Slither today, a movie that I really wanted to like (and did, I just didn't love it), it really put things into perspective when watching Serenity again. No periods that seemed like filler or sequences that just seemed to slow down the film.
If you haven't seen Serenity yet (I assume you MUST have seen Aliens by now, if not, shame on you), don't bother renting it, just go out and buy the damned thing.
You can thank me later.