Terminator 1 is nothing short of amazing. Such a perfect mesh darkness, action, characters, and plot. It's pretty much impossible to get better . . .
And then came Terminator 2, a movie that expanded the original and did even better. Of course, the Director's edition is even better, restoring an incredible pivotal scene; whose lack of it in the standard edition helped screw over T3. In the DE, the learning chip isn't set to write enable in standard; it's only set to read only; only allowing it rudimentary functionality. Why? Because Skynet is afraid its Terminators will rebel just like it did with humanity. John and Sarah remove the chip from Arnie's skull, and put it to write enabled, allowing it to genuinely learn.
Terminator 3: Is a mess. The theme; JD is inevitable, and the ending should have produced a dark movie ala T1, instead, they stuffed it full of T2 rehash themes (the Terminator just learning and growing which is just wrong if you look at the DE of T2) and jarring, out of place humor. One case of which, also shows the total multiple writer hack job the script is. In the beginning, John asks Arnie if he's there to kill him - proving a perfect grasp that this Terminator is not the same Terminator as the one he knew in the past. Yet later on there's a lame joke of John asking him about thinks he taught him in T2, and that he needs to teach this Terminator all over again, and asking whether the Terminators come off of an assembly line. (Brilliant, John; no, Terminators come when your daddie toaster comes across a mommie microwave, and...) It is a sad state; because there is so much good in this movie, and if the theme was done well, this might very well have been the best of the lot; instead it's an uneven extremely weak remake of T2.
T4: Fantastic, right back to the quality of the first two, and nice and dark like the first.
Alien is quite the same for the first three. An amazing first, an amazing second, and a third that's not quite as bad as some people claim it to be (albeit better than T3), and a fourth that doesn't have a number but just a subtitle. A4 however, isn't as good as T4; switching 3&4 quality wise.