I actually thought AVP's effects were quite weak in places. In certain scenes if u look closely u can see the CG aliens hopping all over the place in a completely unrealistic manner, & it completely blows the effect. But onto there suitwork & animatronics. The predator & alien suitwork & costumes were passable. The alien's hands had big rubbery looking fingers that seemed completely out of place with the rest of it's body & seemed rather an extra piece of kit clipped from the muppets or something rather than lethal weapons.
The predators also had their share of visual & effects faults in AVP. In the scene where scar leaps across the tops of the statues & goes right over the heads of the characters, then cloaks. He looks like he is trying to swim or something, & it looks completely ridiculous. The predator's faces also looked horrible. The first time I watched AVP, as scar began to remove his mask, I was expecting to see the mean, menacing bad-@$$ creature that I had been familiar with for so long. But rather, what I got was a creature which somewhat resembled the old design, although it appeared to have a Mr. Bean expression on it's face. Almost as if it was a mentally disabled predator & I was just sitting in the cinema thinking in the back of my mind "oh hell, what have they done to it?"
But anyway, in my opinion the movie that had the best special effects for the time was probably the original Predator. Yes, the actual creature suit was done better for it's time in Alien, but there were alot of things in Alien that blew the effect. For example when the Alien approaches Lambert after killing Parker, it appears it if it is just drifting, it's movements were, literally, snail-paced. Or the scene where Ripley finally managed to blow it out of the airlock but the cable from the harpoon was still embedded in it, technically it should have melted from the acid blood & the alien should have been blasted straight off into space, but that's not the point. The part which blew the effect was when the alien got pulled back in & bumped against the side of the nostromo, it looked like an action figure dangling on a piece of string, & completely blew the effect in my opinion.
The original Predator had hardly any faults on the visual side, the creature itself was excellently done & looked incredibly lifelike, even by "modern" standards of effects, & with Kevin Peter Hall working his brilliance to breathe life into the creature & make it the ledgendary hunter we know today, it's a shame he passed on. In my opinion, no-one could replicate the amount of depth he gave the character, all of his movements were excellently done, he was the definitive predator in my opinion. everything fell into place when the movie was finally realeased. There were hardly any cheaply done effects, everything was spot on.