Aliens is by far the best of the franchise but I actually enjoyed Alien Resurrection and it had a pretty good cast too!
Saw a doco by Cameron on Aliens and he used the Vietnam war as the basis for the plot/storyline etc. A well equipped fighting force overpowered by an inferior (in terms of firewpower), but more determined opponent fighting for their existence. Same as Avatar.
Aliens was done on a very small budget of around $16 million but you wouldn't know to watch the movie. Ranks in my top five all time movies. Cast was great, gung ho characters like Hudson, Drake and Vasquez. Hicks was cool and Burke was a real ass!
I like the directors cut too. Gives more of an insight as to what happened to the people in the colony.