Probably PredAlien for me. At least they don't kill their hosts outright like a Ravager. Plus they increase their health with each kill and automatically heal themselves.
Don't forget, the PredAlien increase their health, i pull this ability to the max, and the PredAlien can have...... 6565 HEALTH POINTS!!!!!!!!!! it's freaking, equivalent for 4 Queens :O and can heal itself, so take ten down, relax, and keep killing