I have about 25 trees in my collection that range from beginner training to 40+ years of training. I began working with bonsai using tropical trees such as Ficus that could be brought indoors when I lived on the East Coast. Since moving to California I have recreated my collection and added a wide variety of species. The types of trees I most enjoy are Olive, Boxwood, Ficus, Juniper of various types, and more recently black pines.
Update: The collection has expanded to about 35 trees. Living in Santa Clara, CA provides an interesting environment to experiment on some species while perfecting the standards. In 2016, water quality became a serious issue causing stress on many of trees. Now in 2017 we end the drought with lots of rain. That has caused some issues but since I did most of my re-potting last year the trees were able to shed excess water and avoid rot. Now I am working to enhance foliage of many of the trees, letting them grow so they can be styled again later in the year.