Definitely a rectangle, like
@The Accidental Aquascaper mentions above...
golden ratio,
rule of thirds and the
Fibonacci numbers etc.
I also really like the
aspect ratio of shallow tanks, they often equate to the
silver ratio, or even wider. It's an aspect ratio we're used to viewing the world through; most TV's and cinema screens have a similar aspect ration so we're already familiar and comfortable with it.
As a scaper it gives me much more room to play with and many more options to create depth and perspective and a more complex environment, which holds interest and attention for longer, and probably makes critters feel more at home.
A shallow tank 60x40x25(h)cm, not a great image, but I'm sure you get the idea...
I find cubes are more difficult to scape effectively...