Your dismissal of Conjuration is also a massive mistake. I can beat any enemy in the game on Legendary difficulty with no clothes and no weapons with Conjuration. Illusion and Conjuration in conjunction with each other is one of the most powerful combos in the game.
Could not agree more with that statement!
The Khajiit is the best class b/c it's a cat. Cats fit the invisibility and sneaking theme of the build, which is the most powerful and useful build in the game (optimized for freedom, justice, efficiency, gameplay, etc.). Cats are also adorable.
Although Schalleh, that is a really good point as well!
Personally, I dont think there is a best build at all. Strongest build? Yeah thats possible. Most Sneaky build? Again that is also possible. But putting 'best' on something this diverse is assuming that your opinions are the exact same as everyone elses. Which is not the case.
I have played sneaky characters waaaaay too much. In my opinion, the 'best' build is something that stays the hell away from crouching or sneak attacking with a bow or dagger.
On a another note, this does seem like an effective build for someone looking for this playstyle.