I equate brutal with viscerality and originality.
I think that Lambert's was the most brutal in Alien because the implied tail rape.
Of all the headbites in Aliens, Ferro's was the only one while flying through the air, which led to a damning crash.
Murphy's demise haunted my thoughts for weeks after I saw that when it first came out in Alien³.
And while the only chestburster to burst in Alien: Resurrection, Dr. Wren's death was the most brutal in my opinion because a chestburster not his own busted out of his forehead.
As for the rest of the 4 films:
Dillon's death in Predator was probably the most brutal because he died piece by piece.
Keyes' halving looked the most brutal in Predator 2, because of the flood of torso juices that followed.
Maxwell Stafford looks to have the most brutal death in AVP: Alien vs. Predator because he was sliced into segments and speared through the heart.
It's harder to pick from Aliens vs. Predator - Requiem because there's so many innovative new deaths, but I think Jesse's is second only to the belly bursting pregnant women.
