From Alien: "In space, no one can hear you Scream" (this line actually gave me more scare when I watched the movie, it gives you the feeling of isolation).
From Aliens: "You know Burke, I don't know which species is worse. You don't see them screwing each other over for a g*damn percentage" (quite deep nad philosophical I might add, and I couldn't agree more).
From Predator: "If it bleeds, we can kill it" (short but effective, never knew why, but I always found something in that line to be very true).
Most of the other movies have good quotes, but they're not that memorable in my opinion. There were many more great quotes from Aliens and Predator, but I picked the ones I liked the most.
"There are five possible operations for any army. If you can fight, fight; if you cannot fight, defend; if you cannot defend, flee; if you cannot flee, surrender; if you cannot surrender, die."