The ultralisk/rhinoceros hybrid "onos". A lot of the game from the alien side feels like "get onos on the field as soon as possible then
up everything's shit", because three of their other five life-forms are kinda crap as the game goes on. Skulks (zerglings) don't get attack upgrades while marines get attack and armour buffs over the course of the game, so killing something with a skulk is murderous. Lerks (the flying bastards) have to get in close to use any of their interesting abilities, which basically just means that they get shotgunned down non-stop. Fades should
be respectable mid-tier fighters, but their HP/armour is too low to let them tank hits and their damage is too low to let them be good stealth fighters so really they're useful as neither.
But gorges have a nice siege attack and onos wreck shit all day err'day.