The ideal room is outdoors, because there are no walls or ceiling to act as reflectors. Obviously, that's not an option so next best thing would be a very large, well-damped room having randomly contoured walls with no large flat surfaces. That's probably not an option either so the next best thing that's reasonable is a room that is well-damped and has no dimensions that are integral ratios of another.

Most times, you have to work with what you have. But damping and multisubs really help smooth room modes. If I had to choose between a golden-ratio room with hard surfaces and a single subwoofer or a square room with panel dampers and multisubs, I'd choose the latter. So you can work with a square room or one with 2:1 dimensions if that's what you've got.