Level 1: Stalla only
Level 2: Schweser only
Level 3: CFAI only
I only had a few weeks to study for Levels 1 and 2 (too busy at work, two babies at home), I knew I would not have time to read everything and practice. So, I only watched the Stalla (for Level 1) and Schweser (for Level 2) videos, and then practiced. That worked. For Level 3, I started in October, read CFAI materials (some sections several times), made notes, and then practiced (old exams, sample exams, etc). That worked too.
My two cents: Design a strategy that is very likely to work for you given your constraints. Of course, time is a constraint not matter what, but it is more of a constraint for some than for others. If you have struggle to find time to study, then go for Stalla or Schweser (to be honest, I do not think there is much to choose between them; the reason I picked Stalla and then Schweser is purely because I went for the cheapest each time - price was another constraint). If you can commit more time, go for CFAI - though you will need to put more effort in putting the pieces of the puzzle together than if you go for Stalla or Schweser who do that for you. To me, what was the key to success was practicing over and over again.