I have a HP X360 and an Lumia 950 for testing, using and sent feedback into the feedback hub.
Which devices are the best one for the Insider Program and which ones have the most issues, are interessing questions. In my Windows Insider Community Germany with over 6k members we have a lot of different devices with different issues.
In my opinion the HP X360 and the Lumia 950 are the best ones with only a few issues and I get the new builds as fast as it´s possible.
I use Surface Pro 3, Lumia 950 XL, Lumia 950 and Lumia 650 for testing Insider Preview builds in different rings. My Surface Pro 3 is always on the Fast ring. And currently my Lumia 650 is in the Fast ring and my Lumia 950 is in the Release Preview ring.