Andrea Canesi, Teacher: mBot 1.0: ideal to teach coding, but also electronics and physics. It's higly expandable, you can create your sensor and use it within the robot. It's based on Arduino, it's easy to find a lot of expansions that can be built by the students.
mBot Ranger: It can be used in an open environment, is robust and can be expanded as mBot 1.0.
mBot 2.0: less expandable, but more precise due to the change of motors.
It has a lot of sensors on board, can be connected to a wifi network and can send to and receive data from other robots or the network