To cover the widest range of devices, the best solution is to make your game responsive:
Set scale to Crop, Scale-Inner (my favorite) or Scale-Outer.
And you will have to get along with Anchor-behavior to make your HUD, menus and etc...
Then when developing the sprites aim to certain resolution... I prefer 1920x1080px or if you care about download size more than on quality aim to 1280x720px, I prefer 16:9 ratio because I noticed that in recent years most of the devices aim to show a good 16:9 HD videos.
If you will make it right, you will make twice the work of testing, but quadruple of devices will run your game.
p.s. Those solutions not so good for pixel art games, pixel art games better played on resolution like 256 × 192 px (as old Nintendo) or 240 × 160 px (as Game Boy Advance), and make the scale Letterbox Integger Scale. (Ratio of those 4:3 and 3:2 accordingly)