Prefer Terminator to T2, also prefer Alien to Aliens. Just my opinion though, and I can understand why people may enjoy the sequels better.
Both are sequels, because they tell both show the events after the first films. However Aliens could have been a stand alone film, but I don't think T2 would have stood well as a stand alone.
Would have been interesting to see how Aliens would have done as a stand alone film, without the background of the first film.