I dunno about best damage dealers, unless the distance is far enough, and your aim is good enough, a charging alien can kill the Marine 4 out of 5 times. same with Predator. Also, Predators have the advantage of stealth. so yeah Marines have the firepower, but their limited mobility and vunerabilities balance things out.
I think the best Aliens players and best team of aliens will probably have the most success in multiplayer.
Overall I think it will be very balanced, only those that fight solo, and not really like a smart survivor, will complain the game is unbalanced because they can't be unbeatable like Rambo.