It's fun seeing these selections and inferring people's playstyles.
My fav. (though it's not the most effective) is Cloud of Death because of how well it scales with pathlevels. For me, there's nothing quite like seeing a D9 Master Lich killing dozens (sometimes hundreds) of troops each turn, with screams and little red numbers floating up from the 40-odd squares of affected enemies. Yes, I know Master Enslave and/or Earthquake would be more tactically efficient.
I also love Flame Eruption for related reasons.
-Max