a t-800 (arnie) would just laugh at a predator for the most part, the only weapon in the predator arsenal that would cause a problem would be the shoulder cannon and possibly the lance. a t-1000 (t2 liquid guy) would just dominate a predator. a t-x (t3:rise) would dismantle a predator and laugh doing it.
now for a terminator vs. alien fight, it gets a bit more complicated. the t-800 would have to rely on ranged attack to avoid the blood, but in close quarters, it would still hold its ground. a t-1000 would be completely fine, the liquid metal alloy seems very resistant to about everything but extreme heat, the damaged pieces would probably just fall out to reject the acidic blood. also, the t-1000 has the handy advantage of laughing off a biting attack. the t-x would be the most dangerous to an alien, the built in weaponry and the t-1000 outer skin would make butchering an alien completely feasible. hell one blast from an arm cannon would make for a spray of acidic mist.