I liked a lot of units in this game, but the for me the best was the Predalien. Easy to create, tough to kill, and inflict major damage.
I love the Ravager to, but its a friggen pain to make. You gotta lay the Praetorian egg, then create a Praetorian, then molt again. Like I said, pain to make. For me at least.
But for some reason, I really liked the Runner... Maybe it was the speed and seeing a group slash appart a group of enemies!