What is the best subject for a beginning painter to paint?
Next time you think about building something.
Stop thinking.
Start.
The choice of project doesn't matter.
If it is worth doing, it will probably fail because things that are worth doing correlate with things that are hard to do.
So maybe it's better to pick something that isn't worth doing.
To pick something of manageable size.
Because an uncompleted important project is less important than a completed unimportant project.
What is is.
Haecceity.