Just watched Primer...man that was fantastic!

I'm also a big fan of Sphere. One of the first DVD's I bought and I still own it to this day.

I loved Sunshine, would definitely recommend it!

Another one that just occurred to me is Final Cut. Possibly more of a drama/suspense but it has some sci-fi elements to it. Also probably not overlooked given the two series and everything but if you haven't seen the original Stargate it's worth a watch.

The Fountain - a visual feast and one of my favorite movies of recent years. It's a movie that rewards repeat viewing.

Prey
Event Horizon

Colossus: The Forbin Project
Silent Runiing
Day of the Triffids

Fortress

First Spaceship On Venus (1960)

Grand Tour

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Luc Besson's Le Dernier Combat

A classic that surprises me when people haven't seen it: Hardware Wars