I recently learned that there is a National Bonsai collection in Washington DC. I had a trip planned for a professional workshop last week, so I had the privilege of seeing the collection on my way home. I tend to prefer deciduous bonsai, and the deciduous offerings are mostly Chinese Elm and Trident Maple, which are very traditional bonsai species. I would liked to have seen more Beech and Hornbeam, but I can't really complain as they have some amazing trees. The oldest one dates from 1625, I saw one from 1905, and the rest seemed to be mostly 1960's and '70's. Here are some of my favorites from the visit:
Chinese Elm, Slant style
Sweetgum, informal upright
California Juniper, 1964
Boxwood, 1975
Common Privet, multiple trunk, 1979
Sargent Juniper, 1905
Chinese Elm, age unknown, windswept style
Japanese Yew, 1966