If you could learn to use only one spell from Harry Potter for the rest of your life, what spell would you pick?

Crucio (Cruciatus Curse)
Pronunciation: KROO-see-oh/KROO-shee-OH (IPA: /ˈkɹu.si.əʊ/)
Description: Inflicts unbearable pain on the recipient of the curse. The effects of the curse depend upon the desires and emotions of the character - to produce the "excruciating" pain implied by the name, one must (according to Bellatrix Lestrange) desire to cause pain purely for its own sake or for fulfillment. The extreme pain inflicted by the curse when cast so - without any apparent evidence of physical harm - makes it uniquely suited as a form of torture. This pain has included the sensations of flaming bones and a head that was split open by crude methods without sedation.[HP4] One of the three Unforgivable Curses.
Seen/Mentioned: first seen in Goblet of Fire introduced by Barty Crouch Jr (acting as Moody) and used on a spider. Used regularly by the Death Eaters as torture, and by Voldemort as punishment, even against his servants. Used twice by Harry on Death Eaters.
Suggested Etymology: Crucio (Latin) means "I torture", originating from crux (genitive crucis), which means "torture platform or stake", or more specifically, "cross". The word excruciating is descended from the same root - crucifixion was a form of torturous execution. In the novels, the verbal form of the word is 'cruciate', as when Amycus Carrow says in the final book "I'll Cruciate the lot of 'em." The caster has to believe the spell will work, as Harry realized in Deathly Hallows using it against Amycus, when he disrespected Professor McGonagall in the Ravenclaw common room.

I feel evil right now, but this would come in handy a lot.