This is the core topic of Gary Keller and Jay Papasan's book The ONE Thing : "What's the ONE Thing you can do this week such that by doing it everything else would be easier or unnecessary?"

Once you determine your 1-3 most important tasks, they are scheduled first in your day. You then make progress on essential items before you get bombarded by distractions. You can use this in conjunction with the time blocking method, saving your initial hours for the most important tasks. Email, phone calls, and meetings come later, after you've completed your essential tasks.

By focusing obsessively on your most important tasks, every day is productive. You never have a day where you waste time on meaningless tasks. Productivity expert James Clear puts it this way :

If you do the most important thing first each day, then you'll always get something important done. I don't know about you, but this is a big deal for me. There are many days when I waste hours crossing off the 4th, 5th, or 6th most important tasks on my to do list and never get around to doing the most important thing.