While I would not call Serenity the best sci-fi movie ever, I loved the film, and am glad that it got some recognition.
http://news.bbc.co.uk/2/hi/entertainment/6517155.stm
If I had to pick the best sci-fi film of all time, it would have to be Forbidden Planet. That movie influenced so much future sci-fi that it deserves the “Best Ever” title.