I watched Alien (original), Aliens (special edition) and Alien 3 (original). I didn't get to Alien Resurrection but I'll probably watch that today.
I picked original Alien 3 because I've only seen it once or twice (first version I saw) yet have seen the special edition one dozens of times. I kinda forgot how different the original one was because I'm so used to the special edition, I knew it would have the other host but there were differences beyond that such as the entire crash landing part (in which case I forgot about Newt's more horrific flooded tube scene for starters), and the movie generally felt faster paced and because I'm used to the special edition it felt like parts were outright skipped and certain events ended differently.
I now feel like before I move to Resurrection today I have to go back and watch the special edition of Alien 3 first (maybe).