I don't abuse, but have Ice Form hotkeyed for those fight that I need it. Now, that comes out instantly, can hit multiple mobs in front of you, and doesn't cost Magicka if you hapen to be a Destruction Mage. That's what i call OP, except for the cooldown, which means you can't spam it. The Alteration Spell takes up a hand, a little while to charge up, and a whole lot of Magicka, which i tend to consider better balanced with the fact that it can be Spammed. Paralysis Poison, if you're doing Alchemy, can be combined with Lingering Poison Damage, so you can just let them die. And, you can apply it in the safety of the Menu (Paused) so the next strike is a killshot. Nasty. The Enchantment effect is the least reliable, except for Paralyzing Strike, which is also tricky to land, and finally, there's Deep Freeze, which is almost Expert level (70 Destruction) but unreliable, and requires them to be low on health to proc in the first place.
Listed in order of relative OPness (If you'll pardon the expression) the spell is second only to the Shout for abusability.