My favorite is "scale outer"
Here the resolution is not that important. The important part is 16/9, 4/3, ...
So I choose normally 16/9 that would work with a lot of the screens
In my sample project you will find a safezone sprite. As long as you place your objects in the safezone everybody will see it - doesn't matter which screen ratio. If you have a different ratio you will get more background (so make your background larger than the safe zone.
Here is a blank project