I bought 2 headphones. ones frequency range is 15-22 000 Hz the others range is 18-18 000 Hz. which is better?

It depends on your ears. You probalby can't distinguish the low end (15 Hz vs 18 Hz) all that well but depending on your age and hearing, you may well be able to tell the difference in sound quality at the high end (18 kHz vs 22 kHz). If your older, like me, you may not be benefitting all that much from the response above 18kHz. "Better" is really subjective since it involves how you hear.