I just found out how to power level your sneaking super fast!
The first thing you need to do is do all the main quest untill you meet Paarthurnax ( The Big Dragon at The Throat of The World)
Whenever he is just sitting at his stone sneak up at him and wait untill you're Hidden, and then just keep hitting him with whateveer weapon youve got.
(you still need to be sneaking while hitting. Paarthurnax will never die nor attack you, so youre completely safe.
And that is how you are going to top 100 in sneaking in no time!
Paarthurnax is located at the Throat of The World, The same place that the GreyBeards are located, you need to unlock the Thu'um (Clear Skies) before you can get to him.



I HOPE IT HELPED!

Meh, bug exploit. You could set the skill to 100 with the console right away -.- I like to level everything naturally. So only when i need to use it, they lvl.

not everyone plays on a PC

you can just hit a companion with a low dmg weapon over and over, ooc hp reg is fast

how is that a fast way to lvl sneak?
doing mainquest to that point takes hours.

There are downsides to getting level 100 sneak earlier on, the enemies that scale will be much harder to deal with if you have sub par gear or very sub par combat skills, but sneak attacks should be fun.