It's really whichever one you prefer. If you get an Emerald be VERY sure that it is not wild caught. They are known to have problems.
Amazons, if you can get them as a baby or still small you can usually tame them down enough where you can handle them. But then you still have some that always bite.
Green Tree Pythons are usually not that aggressive once they are use to you but the babies are very fragile so if you're planning on holding them, a year old or older would be best. You can get farm bred imports for around $175 to $300 that usually do well and c.b. are available too but are a little more expensive.