I have tried everything, from Destruction mage, to heavy armor dual sword, combination of both, one-handed with magic + shout, illusion + conjuration + stealth, etc ... but at the end of the day, in my humble opinion, nothing beats a shadow warrior. You are invisible, undetectable and you don’t need to have a very powerful dagger to kill. For enemies away from you, your bow, while you are sneaking give you the fast killing that not even destruction magic can achieve in one single hit.
Illusion is pure fun; Conjuration is not that powerful; Destruction is powerful but you do not kill in one single hit; heavy armor protects you better than light, but you are not invincible either; dual sword make you kill easier but leave you exposed to mages or necromancers, bows and shouts from the enemy; pure mage makes you dangerous but again, you don’t kill in one single hit, not even if you are able to cast Lighting Storm which is probably the best destruction magic out there.
At the end of the day, shadow warrior make you simply lethal in one single hit, invisible, undetectable, lighter, faster and in combination with archery, you are simply in charge of every single opponent, not matter how powerful they are. They are dead in not time. You probably won’t need magic at all, you won’t probably won’t need to carry health potions and if you combine that with alchemy / enchanting to increase your fortify health, stamina, magic, muffle, light-armor, etc, you are untouchable and absolutely a killing machine in one single hit.
Even if you do not use your alchemy / enchanting to increase what I said, you are still probably the best killer out there because in one single slice or hit, you will kill your enemy and everything comes down to that. You don’t want to hit your enemy 3 or 4 times to terminate him / her, or to use your magic several times, especially when you are facing several enemies and if they are vampires or necromancers, you will not be that happy at all.
I do not want to convince anybody but I just want to support my point : in real combat, a sniper is the most lethal enemy out there, capable to even stop an entire army from advancing, capable to make the enemy absolutely crazy and nut and the sniper in real combat is similar to the shadow warrior in Skyrim. The question they make when they are facing a shadow warrior is : where are you ? That is exactly the question soldiers express when they are facing a sniper which is a shadow warrior with a rifle and in Skyrim, instead a rifle, he is using either his dagger or his bow which stands for the sniper rifle. Just my 2 cents ...