Ice blast or frozen orb for single target?

What is the most efficient spell to be using in between casts of blizzard for boss killing?

Assuming you are using a Blizzard or Blizzard/Fire build, then

Ice Blast would be the spell of choice, as it should have been maxed out along with the other synergistic spells for Blizzard. In this build,

Frozen Orb is often skipped, or have only 1 skill point added (for the convenience of helping rushing low level toons), and won't provide enough damage in high level fights.