Best Aspect Ratio

What is the best aspect ratio for bot orientations to choose before starting to design a game?

I am guessing that the best aspect ratio has to be on that is common for most devices and thus the same view is displayed on most devices.,What is the best aspect ratio to choose

There are two schools of thought:

a) Design the game to look good whatever the aspect ratio of the device it runs on.

b) Design the game to use an aspect ratio appropriate to the genre. A puzzle game might be 4x3, a game that is filmic in nature might be 2.39:1 or 16:9.