I love all. They're hard to compare directly to one another though. Like T1 and T2 are both 10/10 in my book, but for different reasons. As far as all four of these films go though, I can't watch one without watching the other. Meaning, if I watch Alien, I have to watch Aliens (maybe not same day, but within a few days) and same goes for The Terminator and T2. Or if I'm specifically in the mood for Aliens, I can't watch it without watching Alien first. Same with Terminator. If I suddenly desire to watch T2, I have to watch The Terminator first. Both sets from each franchise are intrinsically linked and go hand in hand with one another.
Also, everything in the Alien universe post-Aliens doesn't exist as far as I am concerned (though Alien: Isolation gets a pass because it was amazing). Same with Terminator. Everything after James Cameron's direct involvement is dead to me.