I've seen a few Sci Fi lists and I don't know, sometimes I think a franchise like the Terminator movies started as "Sci Fi" but really just ended as "action" movies. And I've seen people list Predator as a top sci fi movie and while it is awesome and a classic, I never really thought of that movie as sci fi. Like a movie like Stargate is sci fi but is easily digestable so it feels more like an action movie however a movie like Event Horizon is straight up evil sci fi.
Honourable mentions: Mad Max 2, original Total Recall, Children of Men, Moon (starring Sam Rockwell), District 9. I also kinda liked Starship Troopers.
Obscure mention: "They Live" starring Rowdy Roddy Piper was low budget and kinda cheesy but was still entertaining for me anyway.