I'm surprised T2 is the most popular here. Maybe I'm forgetting how good the theatrical cut was because all I have is the T2:Ultimate edition on plain ol' Region2 dvd and that only gave the overlong special edition cut.
While I think Aliens benefitted from its extra scenes I really don't think T2 did. It slowed the film down too much after the pescadero escape and the T-1000 went missing for about half an hour or more - when you suddenly see him again its like 'oh yeah, forgot about him'. As a result T2 lacked the urgency of the first film where it seemed the terminator was always just around the corner. Also there was young john connor. Now, I've previously argued against people who hate this child performance and I stand by that - it wasn't crap, it wasn't great because few child actors ever are - but it was acceptable, understandable, tolerable and it doesn't take me out of the film. However that doesn't mean its my favourite thing to watch. So T2 lost my vote on that one too.
Aliens is objectively the better film IMO - I don't think any of it was something that didn't need to be filmed and put on screen. It was a perfect successor to Alien in terms of making it a different experience for the viewer and yet clearly being part of that same universe as seen in the first film. It introduced some of what would come to be identified as 'crowd-pleasing one liners' that the subsequent films - particularly Resurrection and the AvPs - would become overdependant on for the human characters but it was not yet a pandemic of pandering crap at the time of Aliens. There was still much in the way of dialogue that wouldn't have been out-of-place in the first film - likewise Alien 3 IMO.
Resurrection's characters were cardboard cut-outs of the Aliens guys. Any of the marines who survive long enough you have actually come to like by the time of their demise or in the cases of Hicks and Bishop many of us were outraged at their dismissal before Alien 3 even started. The marines, like the oil rig crew of the nostromo, were a believable unit - believably existing in the same universe as the crew from Alien. This is why I hate Resurrection so much because I just cannot accept that those caricatures are supposed to be considered alongside Parker, Brett, Lambert and Dallas/ Gorman, Hicks, Apone and Burke.
Anyway, I went on a tangent there comparing all the Alien films, point was Aliens carries an emotional punch that makes the action all the more enjoyable etc etc etc. T2 falls down by its pacing etc etc etc