I would have to say a standard assassin. A lot of people write it off as "boring" because all you do is kill from the shadows and remain undetected, and where is the fun in not being in open combat right? Wrong. Being an assassin opens up a new way of thinking and a new strategy rather than weapons out/spells flying style of gameplay.
Basically my main char playstyle, a spell-bow. No, not a bow mage, that is someone that uses a spectral/bound bow. I mean bow and illusion/destruction spells in 50/50 playstyle.
Very rewarding, very fun. You can go invis, and then either turn them to an ice sculpture or shoot them in the head from 200ft. away with a jarrin root poisoned daedric arrow....
A no-kills playstyle without Illusion was one of the most rewarding and fun playstyles in Skyrim for me. Sneaking around enemies, distracting them to slip past them, not to mention countless bosses fallen to the traps and environment. It makes almost any encounter a unique puzzle to be solved. I agree that Sneak is broken, but only when you perk it heavily. I was playing with no perks in Sneak, only using vampire +25% boost.
It means no summons, followers (some quest followers like Barbas don't contribute to your kill count but most do). The Illusion magic is fine but it becomes boring on higher levels. The physics kills don't add to your kill count, so Fus Ro Dah works if the enemy is killed by fall damage and not the shout damage itself. Dark Brotherhood questline requires you to kill 4-5 people yourself, so I destroyed them Was running Astrid over with a cart countless times
That's the main fun of this playstyle, finding ways to dispose of your enemies creatively. Like Hitman games, no one will be able to blame you, muhaha The hilarious part in DB questline was that Astrid is not actually required to kill! I frenzied her when she was lying burnt in the Black Sacrament circle and she jumped to her feet and started attacking! Nazir made quick work of her, lol
I like an agility-based heavy armored stealth style. Pick up the Steed and Muffle, and you can move and sneak very effectively even at level 1 in Banded Iron. Pick up perks like SIlent Roll, rather than Backstab, and use normal swords or war axes, not bows and daggers.
It's cool because if/when you are detected, you are prepared to fight head-on with heavy armor and a decent blade/axe. And since you avoid those damage boosting stealth perks, and instead pick up the movement and agility perks, you have incentive to be aggressive while sneaking, without worrying about being detected. Far more fast-paced and exhilarating stealth approach, while still allowing for a fully tactical style if desired.