I created a special spell just for killing rats and mudcrabs for January's playthrough of Oblivion. It is a Fire spell, 10 points of damage, in a 10 foot area, 1 second duration. I can't remember if it is Apprentice level, or Novice level. In any case, it is really cheap in mana to use, and the 10 foot area effect makes getting those pesky leaping rats easy, as you only have to get close.

If you want it just for rats, you could lower it to 6 points of damage. You need the extra bump to 10 to one shot kill mudcrabs too.

Hee hee that's awesome. Don't cast this spell if Lady Saga's around; it'll kill all her rats!

With a certain mod from a certain author, that spell isn't necessary...