Have to go with Aliens by a fair margin. I expect to be burned at the stake, but I actually like Terminator better than its sequel. T2 was my favorite as a kid, but over time I came to appreciate certain things about the original that I never really noticed before. It's a very deep film with subliminal notions that I sometimes wonder if Cameron himself was even aware of. Probably the same reason some people consider Alien to be the best in the series. Aliens and the Road Warrior are among the few 80s sci-fi action sequels that I like better than their predecessors. Terminator, Robocop, Predator, Escape From NY; all superior to their sequels in my opinion.
Aliens, mainly because it's six years older, and the visual effects still hold up to this day. It's also more influential in sci-fi/action movies/games. Dropships, sentry guns, ammo counters, space marines...
There needs to be a Terminator game where you are relentlessly hunted down. It'd be fun to traverse derelict wastelands, using split or open paths, go into buildings or office complexes being hunted, having fire fights.
An open world setting would be great. It's been done with zombies so why not Terminators?