Thursday Plantation essential oils are pure, high quality and reasonably priced, now even better with 40% off. These are much better quality than those oil sets you get from third-party amazon sellers or eBay that claim to be pure.
You can use it with your ultrasonic diffusers. Lavender is the one I use the most for that, sometimes on its own, sometimes mixed with other oils to make a blend. Eucalyptus is good for clearing airways. If you want to apply essential oils to your skin (massage oil etc), make sure you dilute them with a carrier oil first. Thursday Plantation's roll on products are already diluted if you don't want to DIY.
Amazon has price matched only these so far:
who decides they are Essential ?