Although not all that popular I find that the Guava Bonsai Tree is the easiest to grow and care for.
The Guava Bonsai Tree requires less care and maintenance than other Bonsai Trees to grow and Thrive.
Other good Bonsai Trees that are easy to grow and care for are the Pine Bonsai, Bougainvillea, Dwarf Schefflera, Olive Bonsai, Indian Banyan and the Azalea.
All Bonsai Trees are pretty easy to care for once you get the learning curve down but the above Bonsai Trees are good Bonsai Trees for beginners.