I love Predator 2 for a lot of reasons... The main hunter in it was just more fun, younger and a little more intense.. cooler weapons.. The Leader
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So as a fan girl who loves Predator, of course I have to pick Predator 2.
However, stepping out of the fan girl role for a moment and taking a look at what makes a movie great, I have to pick Aliens.
I loved Aliens, not only did it feature incredible effects AND a great plot, it also had well written and acted very believable characters. The characters in P2 were poor at best, you have the mysterious government agent and his hard jawed sidekick ( just way too sterotypical) and you have the gritty tough cop who's too cool to speak out of monotone and grew up on the streets.
Ironically, of course not only was Paxton in Aliens, he was also in Predator 2, although his performance in Aliens was far superior, I did like his performance, though too. But in Aliens, Paxton really makes that scene and really adds something to the whole movie and created a very memorable line. Way better acting throughout Aliens, much better writing, much better storyline.
P2 has got a lot of plusses in my book and I like the movie a lot, but as far a better film all together, the award goes to Aliens. Personally I think Aliens was the best of the Alien franchise too.