Well actually, the only things we saw about itachi didn't show us much of his strengh...
That's why i didn't talk about him.
But sasuke will definitely become stronger if he chooses orochimaru's path...

So let's drop the "orochimaru is the strongest"...
Well if he's not, and he's being able to kill 2 kages and survive...
The stronger ones must be insane monsters that can definitely control the whole world.
Why don't the villages track them down and kill them all if that's so, because they are a threat to peace!