Define "best". What do you want from the weapon? Best in attack speed, critical damage, weapon type, material, enchantments, set bonuses or just base damage? You can't really say what's best because there's so many possibilities. If you're looking for huge damage, obviously Mehrunes' Razor which can kill in one hit (60%+ chance of doing so), no matter the enemy. Dragonsbane is really good against - you guessed it - dragons, and there's one sword from a Daedra Prince which does fire damage upon hit as long as it's charged. Can't remember what it's called.
I guess it depends on what you're looking for in the weapon, and what your own talent trees are. In previous ES games, there was no "best" - there was just what you wanted and why.