The Warrior. Barring it's ridiculous blue color, it's the Alien that looks most like an Alien. It's also pretty powerful on its own when upgraded. It's got tons of health thanks to the "coming back to life" thing.
I distinctly recall one time where I was a Predator Brawler and was about to attack the Queen. She was alone and only had a single warrior to defend her. So I just send in the Brawler. Since the Warrior was upgraded, it would come back to life and have resistance to "knock back" while on the floor but I figured "Hey, I'm a damn Predator, no stupid Alien can kill me!". Boy, did that Warrior kick my ass.