For beginners and I assume we are talking about first species to deal with in order to have some success and learn as they care for and style the tree, which should be hardy enough to survive the mistakes that are sure to occur....
Indoor: Ficus Retusa, because it responds very well to bonsai techniques while being tolerant of mistakes made. It also has what it takes to make decent bonsai.
Outdoor: Larch, for the reasons listed above.
Will