Also including daggers.
So far, Mehrune's dagger is what I have come up with.
The one you make yourself. Dragonbone plus whatever enchantments you want. Besides that, I think Chillrend is the strongest.
Can it be improved though? If so, that's hilarious considering its a tentacle, but cool.
A self crafted Dragonbone with a dual enchantment of Paralysis / Absorb Health.
The weapons you make yourself will always be better.
DLC and the most unique sword you'll find.
Mehrunes Razor isn't that special. It's the same as a Daedric dagger and the chance of the instant kill is less than 2%. It's a 2 in 101 chance of insta kill.
There are a lot better options than that. No matter if they are found or not.
DLC is part of the game as far as I'm concerned.
but mehrunes razor looks awesome, I concur with the op
The Razor + Assassin's Blade is pretty deadly.
http://i.imgur.com/1dC33yx.jpg
http://i.imgur.com/QI9QZTD.jpg
Miraak's sword has easily become my single most favorite weapon in TES, period. And i genuinely feel sad, upset, and pissed off that I will likely never see it again in another TES game.
I cannot re-call another weapon that even comes close to the uniqueness of the sword. I would love to see it in the next game as Mora's artifact. Or maybe wielded by his high priest or something. I just... want it.
They should definitely make it a reward (or one of the rewards) for future Daedric quest given by Hermaeus Mora.
You specifically asked if Miraks Sword was in the vanilla.
However you want to look at it, I guess.
...nvmd.
Mehrunes' Razor would be my next pick, seeing as how I have a soft spot for Daedric related things, and the razor is a great back-up weapon for a conjurer such as myself.
And it looks awesome.
If you're into a blasphemous sword of a zealous Daedra, then Dawnbreaker is also very brutal... to the undead that is. It can cause AOE fire damage on them. Explosions are always fun aren't they?