Which spells do folks consider worth learning or upgrading ?

[ywhtptgt]

Magic Spring: Good for very long battles where you have a tank stack.
Fear: Low cost disable with long duration if used right
Demon Gate: Excellent fodder and blocking spell (Demons are the best, obviously). If coupled with Magic Spring, it's possible to keep the mana pool refilled very quickly.
Sacrifice: Makes more troops. If you have Demonesses or Hypnosis, can make troops at enemies' expense.
Gift: Very handy for non-Ryders who have rely on Inquisitors for Resurrection in early game.
Invisibility: Essential for many of the cheesy tactics.
Armageddon: If you ever get this spell, it is game over for pretty much any enemy army.

On the other hand, I don't find Sheep very useful simply because it costs an enormous amount of mana and there are other spells out there that can be just as useful.