Mad title I know, but when we explain it, it really makes a lot of sense.
I recently read the book "Eat That Frog" by Brian Tracy, a self-help guru. It's a fantastic productivity title that focuses on the fact that if you do your most important task in the morning, the rest of the tasks you do during the day will seem so much easier - and you will find yourself actually completing your daily task list rather than procrastinating over that task you don't really want to do.
The biggest reason for taking this approach is that you have the most energy in the morning and you also have the most focus. Other times of the day you can find yourself feeling drained, having difficulty summoning the energy to take on a big task. Then there is the worry that comes with knowing you haven't undertaken that task but it STILL needs to be done at some point. Worrying is a pointless use of energy and only leads to negativity. There is no productivity with negativity.
By focusing your highest energy levels on the tasks that require the most focus, you are then taking a very streamlined approach to your day. You are then able to complete your 'low energy' tasks when you are actually starting to flag.
This is why I have set up a Low Energy context in OmniFocus. When it gets to around 3pm, I switch to the Low Energy context and I'm presented with a list of tasks that I don't really need to think about that much. This could be performing backups, making phone calls, filing photo's, standard business admin tasks.
So in essence, do that task you hate the most, or the one that requires the most effort first thing. Eat Your Frog. Once you've eaten that frog, the rest of the day is a breeze!