FanFiltration said:

Films that would be on my list of top 100 that are not on yours:
...
Stargate

He has this one at number 63.

To toss in my two cents (brave of you to open yourself up to a forum with such a list), I personally wouldn't consider Superman sci-fi, but maybe that's just me. :)
One film that does easily qualify for sci-fi that's not on your list is Gattaca, which is a pretty strong sci-fi film that's often overlooked as it's not very actiony, alieny, or fantasy-ishy :)

Thanks for sharing! :)