Again, sorry to fellow Terminator Fans but my money is on the Predator; Terminators just aren't that efficient TBH...
Aliens kill count includes marines, colonists, crew members and civilians and 5 Predators.
5 crew members of the Nostromo, all but one of Hadley's Hope colonists, the majority of the colonial marines, all but one of the prisoners on Fiorina 161, all of the crew of the USM Auriga and most of the crew of the Betty, all but one of (Charles) Weyland's team and the whole town of Gunnison, plus at least five Predators, and a few Aliens.
Predator kill count includes military, civilian, aliens, predators and a Predalien.
2 special forces teams, two rival gangs, most of Keyes team, a few civilians, multiple Alien drones, one Predalien, a bunch of undesirables, and a least one Predator.
Terminator kill count includes a barrage of humans (civilian, police and military), dogs and fellow Terminators.