As you probably know by now, DisplayMate takes display testing to a whole new level, so whilst others are making qualitative opinion statements about which display is best, DisplayMate has the tests and numbers to back up their statements.
So when DisplayMate tell you the cheapest Apple Watch has the best screen, it's worth sitting up and taking notice.
Actually it isn't the screen that makes the difference in this test, as all of the Watches sport OLED screens in two different sizes. The important factor here is the sapphire glass that protects the screen on more expensive Watches.
Apparently the Sapphire reflects twice as much ambient light as the glass on the Watch Sport, which impacts on contrast and colour performance. So if you want the absolute best performance from the Watch display the cheaper model is the one to have.
Otherwise it appears that the Apple Watch has a pretty good display, with DisplayMate's measurements suggesting it meets Apple's definition of a Retina Display. It is also a good colour match to the iPhone 6's display, despite the differing technologies.
Interestingly, although the Apple Watch display is given a very high ranking by DisplayMate it fails to beat the Samsung Gear 2, in part thanks to the latter's use of lew reflection Gorilla Glass. These are the only two smartwatches tested so far.
Definitely worth heading over to DisplayMate's site to get the full details on the testing and performance.