From my tests and I've done a lot of them since I got the phone last month.
8mp in manual mode with my own settings produced the best results in all lighting conditions. Focusing for me is the z2's greatest problem and one which gets worse the more you increase the resolution.
From the watch photo the OP posted, you can clearly see that the 8mp sample is better focused. At 8mp, everything is better, detail preservation, focus, contrast and dynamic range. Yes you can't zoom in as far as you can with the 15 and 20.7mp snaps but the quality is noticeably better.
Sent from my D6503 using XDA Free mobile app