For me, the ideal range of devices would be: a slow device, a fast device, a device under Gingerbread, a device under ICS, a device under JellyBean, a device with a low resolution, a device with a high resolution, a device with a 160 dpi screen, a device with more dpi, a device with a small screen (4"), a 7" tablet, a 8" tablet, a 10" tablet, an Android box for TV, an official Google device (Nexus), a Samsung and a HTC (for the UI layer added by the manufacturer). For budget reasons, devices can match many of these criteria (a slow device under Gingerbread with a low resolution and a small screen).
I bought my devices with that idea in mind. Are still missing: the 8" tablet, the TV box, the Samsung and the HTC devices.