Skyrim - Which is the best of spell in desctruction?

"Best" is questionable.

The "best" spell will only become available to you when you max out destruction, and only through a quest.

But the best destruction spell, the one that will prove most useful, is available much sooner, which is fireball - it will destroy multiple enemies, can be cast at a quick pace, and once paired with the impact perk, enemies will be at your mercy.

Spells become available from mages, such as court mages or those at the college, as your skill grows amd you become able to cast them, so for instance you won't find the fireball spell until your destruction is closer to 50.

The master destruction spells are Fire Storm, Blizzard, and Lightning Storm. You can get them from Faralda (College of Winterhold person) after the quest "Destruction Ritual Spell" is complete, and your destruction skill is high enough.

First off - you need to spell "destruction" correctly.