Here are examples of allergenic plants from the least (1) to the most allergenic (10).  The information was obtained from Thomas Leo Ogren's book, The Allergy Fighting Garden.  I highly recommend purchasing a copy if you are interested in a more comprehensive listing. 

1. Acer rubrum 'Autumn Glory' (Red Maple), Cercidiphyllum japonica (Katsura tree)

2. Cedrus atlantica (Atlas cedar), Chaenomeles (Flowering Quince), Yucca

3. Amelanchier (Serviceberry, Shadbush), Liriodendron (Tulip poplar), Carnations, Dianthus, Pieris (Lily-of-the-Vally bush), Picea (Spruce), Tsuga (Hemlock)

4. Redbud, Night-blooming jasmine, Koelreuteria (Golden rain Tree), Magnolia, Metasequoia glyptostroboides (Dawn Redwood)

5. St. John's wort, Daphne odorata (Winter Daphne), Lagerstroemia (Crape Myrtle), Styringa (lilac)

6. Aucuba (male Gold Dust plant), Catalpa, Female Chinese Tallow tree, Liquidambar (Sweetgum), Tilia (Linden)

7. Boxwood, Carpinus (Hornbeam), Ilex (only male Hollies)

8. Chamaecyparis, Leyland Cypress, Eucalyptus, Gymnocladus (only male Kentucky Coffee tree), Populus (only male Aspen, Cottonwood, Poplar), Quercus (Deciduous Oaks), Thuja (Arborvitae)

9. Betula (Birch), Eleagnus (Russian Olive) Juniper

10. Cryptomeria japonica, Brazilian Pepper Tree, Taxus (only male Yew), Salix babylonica (only male Weeping Willow), Morus alba (only males white Mulberry)