I consider this one to be highly debatable. Someone who prefers the original Alien is invariably a true film enthusiast. I prefer the original Alien, but it's a completely different kind of movie. I can certainly see why the majority of people would prefer the sequel, Aliens. You could argue that the sequel's formula having greater mass appeal means it's better...at many things.
The Terminator vs Terminator 2: Judgement Day isn't so controversial. Again, they are not the same kind of movie. The first movie was pretty damn good when you consider the smaller budget. It exceeded expectations for sure. While big budgets don't always make for great films, the second one far exceeded anyone's expectations and is one of the greatest action films of all time.