AvP 1999 will forever be the best for me
The immersion was completly ass kicking AND the physic was incredible for the time.
I mean that the pred's disc REALLY cut in half his victims (while this isn't true for AvP2 & 3) same for the plasma canon , speargun , wristblades; You were really able to logically put into pieces your opponents
Same for the marines , each bullet impact on the victim was modeled and caused bleeding. Aliens could become some kind of acid showers
I could also talk about the animations , when fighting against an alien you could actually read his movements ,who were varied ( claw attack left then right , tail attack, inner jaw , leaping attack...)
And the lack of story wasn't so bad , it gave the impression of a perpetual chaos & discovery through each map.
Personally this is one of my favorite games of all time.
EDIT: I also like AvP capcom but not for the same reasons.