Round. Why do we waste perfectly good image circle??
I guess I will say I like 5x8. Golden something or other- books have been written on that. I really like squares too. I had a few rough thoughts on these things and put them in my APUG blog post here. I allege that aspect ratio might imply something about static versus dynamic composition.
A very good answer Keith and why is our obsession with Euclidian geometry one that rules out a circular image?