Top notes evaporate more quickly and base notes more slowly. As a result, top note scents are perceived more quickly than base note scents. (This information is from Wikibooks and the links are those provided by Wikibooks.)
There are many many excellent recipes for diffusers online. Just google and trust your nose.
Essential oil
Perfumery note
Family
Agarwood
Base
Woody, Sweet, Resin
Amyris
Resin, Sweet, Woody
Angelica
Herball
Anise
Top
Spicy
Balsam, Peru
Basil
Herbal
Bay
Middle
Bay Laurel
Benzoin
Resin/ Sweet
Bergamot
Citrus
Bergamot Mint
Bois-de-rose — see Rosewood
Top/Middle
Boronia
Cajuput
Camphor
Minty/ Herbal
Cardamom
Carrot Seed
Sweet
Cedarwood, Atlas
Woody
Cedarwood, Virginian
Chamomile, German
Floral
Chamomile, Roman
Cinnamon
Citronella
Citrus/Herbal
Clary Sage
Herbal/Sweet
Clove
Coriander
Spicy/ Herbal
Cypress
Dill
Elemi
Middle/Base
Eucalyptus
Eucalyptus, Lemon
Citrus/Woody
Eucalyptus Radiata
Herbal/Woody
Fennel
Herbal/Spicy
Fir Needle
Frankincense
Sweet/Spicy/Woody
Galbanum
Geranium
Geranium, Rose
Floral/Sweet
Ginger
Grapefruit
Helichrysum
Honeysuckle
Hyssop
Immortelle — see Helichrysum
Jasmine
Juniper Berry
Kanuka
Lavandin
Lavender
Lemon
Lemongrass
Lime
Linden Blossom
Mandarin
Manuka
Marjoram
Middle/Top
May Chang(Litsea Cubeba)
Melissa
Herbal,Lemony
Momordica charantia
Myrrh
Myrtle
Myrtle, Lemon
Neroli
Niaouli
Nutmeg
Oakmoss
Earthy
Olibanum — seeFrankincense
Orange, Bitter
Orange, Sweet
Oregano
Palmarosa
Parsley
Patchouli
Spicy/Woody
Pepper, Black
Peppermint
Minty/Herbal
Petitgrain
Floral/Woody
Pine
Ravensara
Rose
Rosemary
Rosewood
Floral/Spicy
Sandalwood
Sweet/Woody
Spearmint
Minty
Spikenard
Spruce
Tagetes
Tangerine
Tea Tree
Thyme
Tobacco
Tuberose
Turmeric
Vanilla
Vetiver
Violet Leaf
Yarrow
Ylang Ylang
Image: Sonia Orts