I test a lot of devices, but one that I have yet to get my hands for an extended period of time (I played with one briefly at CES) on is the Samsung Galaxy Note, but according to Samsung over 5 million of them have been have been sold. Given the worldwide release in October, that's an average of 1 million a month and quite an achievement for a device that is bigger than most phones and smaller than most tablets.
The Samsung Galaxy Note has a gorgeous 5.3 inch 1280x800 Super AMOLED display that alone makes me want to go buy one. It sports a 1.4 GHz dual core processor and runs on LTE on AT&T. The S Pen stylus is one of the unique aspects of the device and given that we see they have now shipped 5 million units the device seems to have claimed a niche in the mobile market. If I didn't already have an HTC Flyer and PlayBook, I would like pick one of these up and may not be able to resist much longer. If it ran on T-Mobile I would be all over it.