[Erydian]

In no particular order, and these include bard instrument effects:

Eye for an Eye (great for reflecting Death spells)
Insanity
Death Cloud
Draining Cloud
Elemental Shield
Soul Shield
Freeze All
Asphyxiation
Death Wish
Haste
X-Ray
Armorplate
Razor Cloak
Tsunami
Firestorm
Whipping Rocks
Cerebral Hemorrhage
Mind Flay
Heal All
Rest All

These are the spells I found myself casting most often with the following party:

-Dracon Fighter
-Elven Bishop (who finished the game with level 7 spells in all books except Divinity)
-Hobbit Bard
-Mook Ranger
-Rawulf Priest
-Human Valkyrie
-* Vi (most of the time)
-* RSF-81 (for the majority of the game after I found him)

I feel that psionic spells are especially effective towards the beginning of the game, where the majority of the creatures you encounter seem to have little to no mental resistance. This doesn't necessarily hold true for the end of the game (where I tend to favor the alchemist spells), but I still found myself using them from time to time.