strongest spell in skyrim?

Answer 1

maybe fire storm in the destruction tree

it does an explosion of like 100 damage

see list of destruction spells

Answer 2

The strongest is the Ice storm spell.  It can one-shot 20-30 imperials at once with 150+ damage in AOE.  The cast time is around a second, and when dual casted is the deadliest thing in Skyrim.

 Unless of course you run into a frost dragon.