Which is the best, most completed and powerful Arduino?

I am looking for the best Arduino board.
I mean: I don't want to buy a "starter one", or "intermediate one", or "advanced one", and so on.
Just, I am looking for the best, the most completed one.
If there is not (some companies have products with some features which make product A better for X applications and product B for Y applications), please, advise me on which are the most powerful ones and for which kind of projects each one is better.
Can you advise on this, please?
Thanks in advance!

#define best ?
#define complete ?

depending on your needs an UNO can be the best/complete. It really depends on your project(s).

From price performance point of view I advice (mostly) to buy 2 UNO's instead of 1 MEGA as they can communicate in different ways.

The DUE, Teensy3 and the YUN are probably the most powerful but in different ways.

You have to define what your application is. You may as well ask which is the best car or best aircraft. Or best school. Or best food. Or anything really.