What are your favourite skulls? My most common choice is the "Hollow", followed by "Blind". Sometimes I take "Black Eye", especially in missions where shield regeneration armour feature is available, as well as in vehicle based missions, though in both cases it is a little bit of a practical cheat. The most difficult one I think is the "Pacifist", as practically it requires melee only run or capture the point neckbreak type of run, which is ridiculous in terms of swarm defense scenarios. But maybe I miss something in the logic of this skull. "Famine" is surprisingly difficult, though in vehicle based missions I can see it as much a practical cheat as the "Black Eye" is, although weaker in terms of experience boost. "Thunderstorm" I would hesistate to pick up without "Damage Booster" upgrade, nonetheless "Thunderstorm" is still less valuable than "Black Eye" as a companion to "Hollow" anytime choice. Generally I always try to complete a mission with some skulls on.