The most popular devices seem to be the Amazon Fire tablets, Apple iPad, and Samsung Galaxy Tab series of tablets.
Personally, I use Fire HD 8 and Fire HD 10 devices for my dedicated tablets along with Fully Kiosk Browser which has some nice features for motion detection, screensavers, etc.
Side note: From my experience, the Fire HD 7 tends to be underpowered, so I would stick with at least a Fire HD 8 if you go with an Amazon device. The bigger devices tend to have better specs and are more powerful.
Here’s a few photos of my old setup (nearly a year ago - I need to snap some new photos!):
And I use SharpTools dashboards on my Pixel 2 XL phone using the Add To Homescreen approach.