Recommendations for Sci-Fi movies

I know that this has been done to death in the past, but I am facing a dilemma in that some recently released movies (“Moon” for example) which received rave reviews was totally boring and at the end of it I was not even sure of the plot (theme).

So I am looking for older movies that have been tried and proven.

Obviously not all recent releases are bombs. Avatar was awesome. But the online reviews I read rarely coincide with my idea of a good Sc-Fi movie.

All recommendations, with reasons, will be gratefully received.