-Marines, simply because you can identify with them as living beings. The Weyland Yutani Corp is just a company. Granted, a company who is putting many lives at risk by attempting to capture & use Aliens as bio-weapons, but still just a company nonetheless.
-I am quite partial to both heavy & slow, & light & fast. Depending on the prey. A large, powerful Predator could fight off Aliens much easier than a smaller & quicker one, as no matter how quick it would be, the Alien would almost certainly be able to outmaneouver it, wheras a light & fast Predator would be extremelly effective while hunting Humans, as it's great speed & maneouverability would allow it to quickly rush in & kill it's prey before they could react.
-Warriors. In my opinion they are the greatest of the adult Aliens. They may not have the speed of a Runner, of the physical power of a Predalien, but they have a very balanced combination of both attributes. Combine that with a hardened exoskeleton, able to deflect low-caliber bullets, & an extreme level of intelligence, & you've got yourself one deadly killing machine.