The best thing about master studies is that all the creative decisions have been made by a master. Your job is to process their decisions: their composition, color combo, color contrast, shape design, how they simplify anatomy, where they place their shadows and highlight, how they transition their shadows (hard and soft edges) etc. Ask yourself why they made the decisions they made.
When it comes to picking a piece to reproduce, you might consider what painting techniques/ art style you want to learn.
John Singer Sargent, for example, uses ala prima (wet on wet) technique, which is all about layering wet paint ontop of each another and using an economical amount of brushtrokes to express as much as you can (brush economy). Every brushstroke is a suggestion of a detail. As a result, his work is really loose to the point of abstraction at times.
In contrast, Bouguereau does detailed underdrawing and a lot of detailed blending with a soft brush.
Vermeer uses a more lengthy process of gray underpainting and then layering on transparent color glazing.
It comes down to how far you want to take it. Technically, you can copy any of these works with any technique, or even pencil drawings to study things like shape design and composition etc. You can also replicate their techniques to a point with digital art, but you won't know how to mix paint or the feeling of how real paint might interact with eachother.