The best race has to be the Predator becasue they kiss the ass of marines and can take most crap from aliens.
Not as a matter of course. For example: Marine + Smart gun = dead Predator or, an alternative is to grab the grenade launcher, plenty of ammo and hole up in a strong point with only one enterance, which you plaster with mines. Yes, the Marine isn't as tough as a Predator, but pound for pound he comes with a bucket more firepower.
All that said if I'm playing mutliplayer, my persoal favorite has to be the runner alien, it does less damage than the other breeds, but is a lot faster. When playing aginst human players you have to close the distance on them really fast, and the runner excells at that.