My preferred shape for a dining room table is round because at a rectangular table you only can visit with the people next to you. At a round table, you can converse with everyone; you also can seat more people “in the round.”

If your room layout demands a rectangular table, consider one supported by a center pedestal or trestle. The lack of legs at the corners increases your seating options, allowing you to position chairs all around. - Candice Olson from the book:
Candice Olson on Design