I appreciate and respect the choices so far, but Alien has it more inferred and that's very effective but only one 'burst' of gore. Aliens is more or less devoid of any 'gore' at all really.
Predator 2 is more splatter and dismembered set peices and slightly comic book, Alien 3 was pretty fake looking and the low grade bluescreen Alien shots didn't help convince me either that the threat was 'real' and Resurrection was, in my opinion, a comic book with some 'cheesy' unconvincing gore.
AVP was tamest, but only in terms of human gore, Alien and Predator gore was damn good but as much as I'd like to, I can't use that as a comparison.
Predator 1 on the other hand is old school combat violence with some shocking and visceral moments
that really do put it a class above all mentioned in the 'real gore' stakes.