Predator 
This is what is is all about for me. Sci-fi, horror, action and war all rolled into one with plenty of great one liners from our man Schwarzenegger ("Stick around!"), totally in his prime. From the opening sequence in the chopper: "My wife said she wanted a little pussy!" to the systematic killing-off of each member of the team, Predator has it all.
The Predator creature has gone on to have its own mythology with sequels and comics, and a believable mythology always improves a science fiction movie and keeps you thinking outside the celluloid.
And not to forget the greatest handshake of all time:
"Because some damn fool accused you of being the best!"