When I mix it up myself: peppermint, tea tree, rosemary, lavender, white thyme, atlas cedarwood.
Citrus oils (except tangerine) send my scalp to the fiery depths of hell so I don't use them. I usually dump extra lavender in if I'm not feeling the scent that day but I usually don't mind since my moisturizers are heavily scented and you never really smell the scalp oil anyway.