I'm new around here (and to this game), so I'm not sure what you mean by "no spell stacking" (but I'm probably about to violate that). I found this thread when looking for pages on people's favorite custom spells, though, and I notice nobody else has chimed in yet, so I figured I'd try to get the ball (re-)rolling.
Exhaustion
Damage Fatigue 50 points for 2 seconds on touch
Weakness to Magic 100% for 2 seconds on touch
Requires: 50 Destruction
Notes: For those who don't specialize in Illusion magic but would still like to knock foes down, this spell can be even more potent than Paralyze effects, if a bit more risky. You need to cast it multiple times in quick succession, but each time you cast it, the effect is magnified until your opponent's Fatigue reaches zero, knocking them down helplessly. It works even better when paired with some other item or effect that Drains fatigue or further increases Weakness to Magic. When I reached 75 Destruction, I raised the damage to Fatigue and added an effect to Drain Fatigue 100 points for 5 seconds; at level 30, two shots will take down a brown bear for at least 5 seconds, giving me plenty of time to practice Hand to Hand (which further damages Fatigue) or Marksmanship on a helpless foe. Recommended for use with high Speed so you can get out of the way between casts.