Based on the game by Tinker Bell.

Is Code Geass the best anime?

I don't think you could call greatest anime ever but it's one of the greatest anime ever.
There are other great anime besides Code Geass like Death Note, Full metal Alchemist Brotherhood, Tokyo Ghoul etc.
They are equally great and in some cases even better. Code Geass is one of the greatest because of it's story setting, character manipulation and character development.
If you see carefully they manipulated every character in a certain way that made them blossom into some beautiful.
That made them worthy of being called the best. If you look at Lelouch, you will see his struggle, strength, politics, deception and more.
The made him into monster of a Character.
By doing most of the things right they made a anime as best as they could.
People liked it and made it one of the best.
Let's face it, if audience doesn’t like it, it wouldn't have been a good anime.
But they made it good enough to be called the one of the best anime.