For a large room, the URPOWER Ultrasonic Aromatherapy Essential Oil Diffuser is a great option. At around $25, it has a 500ml tank, the biggest on the list. It’s a humidifier and a diffuser that can be used for 10 continuous hours. If you want to use it in shorter bursts, it has timers for one, two, or three hours. The only drawback to this model is that it needs to be cleaned once a week. Most models are low maintenance and require little to no cleaning, but if you don’t mind some cleaning, this is the best essential oil diffuser for a large space.