I use a mix:
Frost, not used very often. When I do use it it's to slow them down and drain their stamina.
Shock, usually to sap their magicka and for big hefty damage.
Fire, if I want a heavy dps spell to do the job.
Overall, I dual wield shock and fire for vampires, usually a dps fire spell in one hand (like flames or wall of flames) or maybe even a fire rune, and use a shock spell such as thunderbolt or Chain lightning (depending on if I use fire rune or not for maximum damage). 'Tis so pretty when pl*ps hits the fan! >:3 Ashes to ashes... and dust to dust..
Use fire for bandit mobs, maybe a little frost here and there if there is a big guy running around. We know how THOSE guys are.
Plain ol shock I use for pesky mages. Fight fire with fire as the saying goes.
Frost, slow those tanks down a notch. Especially if lightly armored. Might need to make a run for it if things go bad.
I do NOT use:
Frost on Nords or frost atronachs for obvious reasons. Same goes for vampires. Also do not use it on frost dragons.
Fire on fire atronachs or Dunmer for again, obvious reasons. Also do not use it on fire based dragons
Shock is an all rounder. All except for Storm Atronachs.
Shock is perhaps my favorite because it is an all rounder. Very few are resistant to it except if the target wears enchantments to ward off shock spells. It's very versatile and has a good selection of spells for most situations. In a few situations however, it's best to combine them with other destruction spells or even other spells from a different school. That's what I like about shock, versatile and very open to flexibility. Not to say frost and fire aren't that with other school spells. But shock is just... very well grounded and safe. (Which is Ironic because ground... lightning... yeah bad joke.)
Except if you have followers... Fireball or Chain lightning? Better not use those. Because if you use them and hit them in the arse, you're not going to be looking at a friendly pillow fight. Lydia and others HATE having their butt spiced up with a little blue lightning or having it roasted like a chicken on a spit.