Force field use taunt with your tank then keep force field up on the tank, works every time. ;)
 
Beat the high dragon at lvl 13 with hard difficulty without breaking a sweat... 

Cone of cold is probably my favourite so far. Incredibly useful and satisfying to use.

I love using cone of cold, then stone fist, throwing a rock at a frozen target and seeing them shatter!

As long as we're doing favorite spells and not most effective/efficient I will go with inferno/blizzard/tempest I just feel so awesome when I cast those.

Paralysis/Repulsion.  I like waiting till the melee-ers start brawling then setting it just right to get the enemies, but far enough to miss my own guys.

I don't know why, but my favorite is Life Ward... it just always seems to get the job done. 

Blizzard, love disabling 8+ mobs.

From what I've got so far Inferno is my favorite. It's pretty situational thanks to friendly fire, but it's a great feeling when I get to burst that baby out. I especially like doing it on enemies that haven't noticed I'm there yet.
 
As for useful, I'd propably go for Sleep. Great crowd control.

As someone mentioned Cone of Cold followed by Stonefist = shatter is very satisfying. 
 
I also like the Fireball as it messes dudes up, although it makes the enemies come after Morrigan pretty hard after that

Cone of Cold is THE spell for me.. Once an enemy is frozen, any critical hit will result in shatter. Ogres, dragons (not high dragons), darkspawn.... they all go down..hard.

Cone of Cold is probably my favorite, though I do like the blood mages abilities as well. Still nothing quite compares to shattering your enemies. Smashy smashy :)

I wish they actually shattered though, as in, fell into pieces.

Sleep + Waking Nightmare, can take care of a room of 20ish enemies without you being touched, only downside is since they kill each other you don't get any xp for it.

The spell that gives you an invalid Blood Dragon Armor key is INSANELY awesome.

Seriously, though, Death Hex or whatever is pretty awesome. Allows my 3-tanked party to wipe the floor with bigger enemies.