Which single spell is most powerful for you? As you use it, not in general. Which spell when you cast it will instantly turn the fight in your favour / or simply end the fight there and then?

I'm currently playing a sword and board warrior. He uses Whirlwind Cloak to turn the odds in his favour when fighting more than one enemy. It's an incredibly powerful spell, like a passive unrelenting force. The enemy takes quite some time to get back to their feet after being flung by it, which gives you plenty of time to dish out your damage, or heal up if needs be. It even works on horseback, which actually made me sad yesterday when I killed a deer by flinging it off a cliff.

It's also useful when I'm wielding my crossbow (secondary weapon) as there's a chance to fling the opponent away if they've advanced on me, allowing me to finish them off with a couple more bolts without having to pull out my sword.

Dual cast incinerate/thunderbolt/icy spear with the respective dragon priest mask to boost its damage (with augmented [element] 2/2 they will deal 235 damage per dual cast, and that's without fortify destruction/shalidor's insight)

Too bad I can only practically use these when under the effect of secret of arcana or tree stone power, cause I don't really like zero-costing destruction to cheese my way through everything.

My favorite Spell is Lightning Cloak. I enjoy watching Thalmor Justiciars suffer helplessly as They can not cast even a simple healing spell.

Stendaars Aura, Flame Cloak, and Frost Cloak are used on many of My characters, despite the Elemental Cloaks increasing the Bunny Count

Dual-casted lightning cloak is indeed awesome. It feels like your very presence reduces other mages to iron-dagger wielding weaklings.

Frenzy, Invisibility, and Pacify. I would go through entire dungeons without being hit or noticed.

Fireball. The cost is low enough that I can repeatedly cast it and it will usually be my go to spell for foes.

This would generally be whatever summoned "helper" is available to my character, at any given level. An atronach cast between yourself and your enemy works wonders.