Electro-Mechanical Batteries have many important advantages compared with chemical batteries, especially in low earth orbit satellite applications. Different slotless, external rotor, permanent magnet machines are suggested and used in these systems as Motor/Generator. Synchronous permanent magnet, brushless DC and Halbach machines are three types of them that are most commonly used. In this paper, the best machine will be introduced using a comparative study. Proper material and structure for space applications are introduced and simplified. The analytic design methods are given for three types of machines. Then, all three types of machines are optimized in order to have a fair comparison. Particle swarm optimization is used as the optimization algorithm and the finite element-based simulations are used to confirm the design and optimization process. Finally, the best machine is introduced based on the important parameters in a micro-satellite.