Pick Power Magic EZ, Power Magic Pro, Ultra Battery, Battery Pack (in that order)
Power Magic EZ and Power Magic Pro are a tie for most stealthy installation in the majority of cases but it will vary with each car configuration. If your OBD port is hidden inside of your fuse box, the PMEZ's smaller size makes it easier to hide. If your OBD port is somewhere else, like right under the steering wheel, chances are that the PMP will be easier to hide as you can usually put it in the fuse box.
The Power Magic Ultra Battery, in spite of its larger size, can be stealthily installed as well, as it is able to connect to the fuse box (recommended installation) and tucked under the driver's or passenger's seat or glove box.
The Power Magic Battery Pack is harder to hide since it requires to be connected to a cigarette lighter socket.
Also, if you want to use the USB ports of the B-112, you will likely have to keep it somewhere in sight. However, if your car features some inconspicuous cigarette lighter power socket, the B-112 can be both handy and stealthy.