Since essential oils are all about scent, whichever oils smell good to you together can be considered a good blend.
Lavender is a lovely oil that can add a grounding note to any blend, so always try your favorite blend with a little lavender.
What essential oils can be mixed together?
Lavender and orange make a lovely blend, as well as tea tree and peppermint.
Lavender and frankincense is a great blend to help you drift off to sleep.