I think the Sniper is by far the best Common Survivor. In my opinion, it's a waste of resources to level up any other Common Survivor because there are much better Uncommon, Rare, and Epic units. However, the Sniper is a viable option because he is able to deal a massive amount of damage and if he's leveled up to 30 or more he's able to take out practically any Zombie unit regardless if they have armor or not.