Pacify is the best spell in the whole game! I love Illusion!
My assassin is level 63 right now, still so much fun!
I power-leveled through the early levels:
1) Got Faendal as a follower, trained Archery (to 50), traded my money back, saved all perks.
2) Pickpocketed Amren in Whiterun after One-Handed trains (to 50), saved all perks.
3) Sneak attacked Hodvar in Riverwood (with Thief Stone and Well Rested to 100), spent perks on Sneak tree after: Stealth x 5, Backstab, Deadly Aim, Assassin's Blade.
4) Got Mage Stone next, went to Whiterun and bought Illusion spells (Calm then Muffle) and cast them on or next to a guard over and over. Spent perks in Illusion (most of the tree except Master Illusion).
5) Trained and pickpocketed trainers with a full exp bar (so you can train, then level, then train again right off). Did this every level.
I prioritized my perks like this: Sneak if possible, otherwise Illusion, otherwise One-Handed, Light Armor, Alteration (for Magic Resistance perks), Enchanting, Smithing, and I dabbled just a little in Restoration (Apprentice and Regeneration - I was thinking of Recovery but it wasn't necessary).
As time's gone by I maxed out Speechcraft (by using Persuasion on the Black Briar guy), Alteration (with -100% magic cost and Detect Life in a crowd), Conjuration (-100% magic cost and Soul Trap on a dead body). Extra perks have gone on the left side of the Speechcraft tree for convenience and quicker money and Unhindered. I took 5 Destruction perks for fighting Dragons (see below).
My gear is 1 x Crafting Set (Fortify Alchemy and Smithing), 1 x Illusion Set (-100% cost Illusion, Waterbreathing, Fortify Health, Fortify Stamina, and Fortify Sneak, plus Shrouded Gloves), and then 1 x Dragonslaying Set (-100% cost Destruction and Conjuration) I took Impact and 2 x Shock perks to spam Lightning spells and staggerlock Dragons. I have a Dragonbone Dagger with Absorb Health and Fiery Soul Trap along with The Black Star. And I have Ring of Hircine for my Werewolf transformations.
In short, I can kill many different ways, depending on my mood
I had trouble playing as a Werewolf at first until I got the knack of doing a sprinting power attack and now I've decided Werewolves actually are pretty powerful!
By far the most overpowered method though is dual casting Pacify and one shotting everything with Sneak Attacks.