Alien vs. Predator:
First, I note with surprise that there are only 12 hits for "alien vs predator" "slash fiction".
The effects are good. When Alienses are actually fighting Predators, it's pretty spiffy. Pretty much everything else sucks. They waste 30 minutes introducing you to boring characters who won't ever get another useful thing to say or do for the rest of the movie; the setup is contrived, the ending is stupid. Run towards the camera in slow motion!
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I hate that it took place on Earth. This completely undermines one of the best moments in Aliens: "If just one of those things get down here then that will be all! And all this bullshit that you think is so important, you can kiss all that goodbye!" Oh, but oops, they were here all along and nobody noticed. Way to invalidate Ripley's whole motivation. This pissed me off almost as much as how the killed off Newt before the credits in Alien 3 (AKA "way to invalidate the last half of the second movie.")
They fucked up the life cycle: the aliens go from egg to face-hugger to chest-burster to full-grown in literally ten minutes. Not only did that cycle take almost two days in the previous movies, it just drives home the whole question of where the hell did all that mass come from? With a longer life cycle you can guess that maybe there was time for it to, you know, eat something.
At the end (and presumably this was some ham-handed sequel bait) they leave an Alien Queen alive on Earth! Oh, but she sank in the Antarctic ocean. Yeah, I'm sure the water will hurt her in a way that hard vaccuum didn't. Again, way to invalidate the setup of Aliens.
Moving walls: just plain stupid.
I haven't read the Alien vs. Predator comics (I could smell the stink on that shit from a mile away) so maybe some of this made more sense there (but I doubt it.)
I enjoyed it while it was on, because big monsters hitting each other are cool, but it left a really nasty aftertaste. This is no Freddy vs. Jason. Hell, it's not even as good as Alien 3. It's probably better than Predator 2, though.