I can't say that I agree with this list of the top 100 sci-fi movies at Rotten Tomatoes, but it's decent enough as such lists go. I've definitely already noticed a few I haven't seen and probably should, and a few I should see again.
I was disappointed to find Serenity at number 48, though. There are plenty of clearly inferior flicks in front of it. I would have had it in the top 15, at least.
Update: In fairness, this is based on overall reviews (not sure if just on Rotten Tomatoes or elsewhere), and so it's not per se someone's judgement about where any given film should appear.