For me the best picture quality came from a zgemma. The flexibility on setting resolution to match source was first class. Looking to try a newer model. In the down side the menu was dire and the box keeps locking up. Later boxes have more ram and flash memory.
Shield pro is hyped as the best, it lasted 4 evenings with me. Picture in all settings was over processed loads of ringing and artifacts hated it. The box was fast and loading apps simple. The remote was a pain, half the time i could not get it to move the cursor
Fire Stick, best buy for me fast, easy to use. Reasonable picture if left at 1080. Refresh match never worked and 5.1,dd out took a while ti get working, player dependent. Pain to load apps but downloading to phone and porting over helps
Just bought a fire cube, doubt it will be much better but 64bit processor and extra ram might give it a small boost over the stick