My favourite card base shape is definitely square, usually a 6x6 inch.
There's quite a few reasons for this but mostly it's economics.
It takes a 6x6 inch paper to fully cover a 6x6 inch card with no waste. An A5 sheet of paper covers the card with a neat strip leftover that is great for layouts like this one. The leftovers from an A4 sheet are perfect for most of the little details.
It makes me feel all warm and fuzzy to use up every scrap of my paper without any waste.
I also find that a 6x6 card neatly fits most stamps with just the right amount of room for embellishments.
Here's the sketch from Tuesday Morning Sketches: