So good. Incredible tone and atmosphere. No other space/horror flick comes close, including favorites Event Horizon, Jason X, Freddy in Space, Pandorum, etc.
Aliens is another 10 of course, but loses the horror edge to a degree much like T2 as @ZeldaFreak mentioned. And similar to Terminator, I prefer Alien over the sequel by a slim margin for the horror aspect and sense of impending dread.
And not to mention, peak Sigourney Weaver.