The best low level spell is the simple Drain Health 100 x 1 sec and Soul Trap x 1 sec (on touch).
It uses up next to no magicka and will one hit anything with less than 100 hp. Great for killing trolls!!
Once enemies get over 100 hp I normally progress to a spell like the one UnclePain describes. Or just level up my blade and block while I am at it...
That is indeed the most efficient spell I've used. At higher levels, I added a 100% Weakness to Magic effect in case the first touch doesn't finish off the opponent.
I'm fascinated to see how many posts here combine fire, frost, and shock damage into one spell. I guess that just means you have to switch between spells less, but when I used this, I got so tired of a third of my damage disappearing to opponents' immunities that I made custom versions for each element, each with a little quirk to make things interesting: my Frost spell includes a 1-second Paralyze effect, my Shock spell is the only on-target and area-effect spell (as shock damage hits its target fastest), and my Fire spell does the most damage...
My favorite spell right now (which I just posted to another thread because I didn't realize this one had seen more activity) involves Damage Fatigue, Drain Fatigue, and Weakness to Magic 100%. Relatively cheap, and two casts is enough to down most enemies for longer a single comparably priced Paralyze spell at level 30. Nice if you have been boosting Destruction instead of Illusion and want to practice punching/shooting arrows at a live target.