controlling water would have it's advantages, as would creating ice, you'd never have to walk again. perfector would definitely have it's advantages, balzac could be very useful, shambhala would allow you to go anywhere in seconds. susanoo would help out a ton, with his ability to do household chores so well. i guess there isn't a definitive most useful imperial arms
Also, if the Imperial Arms actually existed, they'd either be used exclusively by the Japanese empire, or be used by people working for governments or the military. Hell, we wouldn't even know about their existence anyways as they'd be top secret.