Dominate or charm person if it changes dialogue and enables some interesting options. It's OK if it's implemented in the Bloodlines fashion.
Invisibility, great for exploration. I particularly like invisible Ogre Magi - these appear very threatening.
Mirror image limited to self. An alternative to wearing armour, enabling interesting close combat builds.
Teleport other, particularly in more vertical games. Adds a tactical layer and removes some of the need for tanking.
Create wall or force field. Worked particularly nice in Underrail, really lets you control the field and it is hard for the devs to account for it.
Summoning spells may be boring, but they are often essential when the tanks are not enough. Summon energy vortex was interesting in Ultima Online because it would attack everyone.
World spells, like in AoW2. E.g., cast a wall spell once to have a permanent wall in a city. Turn a spring into healing waters.
Flying and levitation, obviously, but these tend to be completely overpowered. Also jump and feather fall as in Morrowind. Helps it all feel like a dream.
Spells that knock others down or root them in place. Especially fun in PvP. Also close combat spells.
Pre-casted Kal Vas Flam and a halberd hit.