This is one of the important aspects of GTD that catches a lot of people out the first (few) time(s) they try it. The purpose of the weekly review is really more to audit than to plan. You're looking back at what you have and haven't gotten done and identifying why some things happened and others didn't. Then, you change your system as necessary to address any issues and move on.
It's not really about prioritizing what to focus on in the next week. That's not to say you can't do that to some extent, but you really should avoid making too many decisions about priority in advance. If you are prioritizing your actions, try doing it in the morning as part of a daily review where you decide what you want to get done today rather than trying to plan out the whole week (if I'm in a real rut for whatever reason, I'll choose 3 things in the morning that must get done today and another 3-5 things that I'd like to get done if I can).
The goal is to achieve that "mind like water" state where you're simply reacting to the circumstances of life as they arise. You don't over plan -- instead you adapt to the current situation, and try to make your decisions about priority in the moment rather than in advance.
So, the purpose of a next action list is not to tell you what you should do right now, but to tell you all of the various things you could do right now. Due to certain terms being very overused in GTD apps lately, the concept of next action has gotten a little confused. Some people tend to think that the actions that are highlighting purple as they complete actions in sequence are their "next actions" and that's just not right. Every single action on your action list that is currently active (so anything that is either the next step in a process, a parallel action or whose start date has passed) is a next action.
The next action list tells me everything that I could be doing right now, and it is my responsiblity to decide right now which of those actions takes precedence. Categorizing by context helps narrow down that list considerably since I'm likely only in one to three contexts at any one time so that's the first step in making that decision. The second step... well you have the book, read the chapter. Basically, I'm deciding what I have the time and energy to do in the current context and I'm deciding what is important right now.
I can't make that decision in my weekly review because try as I may, I can't predict how much energy and time I'm going to have Wednesday afternoon and I might not even be able to predict what context I'll be in. So I have to make decisions about priority right before the moment of task execution. It's a hard mental shift to make at first, but once you do it is incredibly liberating and it wasn't until after I started to do this myself that I understood why the next action list is considered by many to be the core concept of GTD.
To put it another way, Steven Pressfield writes in The War of Art that the professional has to learn how to distinguish what is urgent and what is important, and then they do what is important first. Priorities, end dates, flags; they're all just ways of making certain things urgent. What you need to do is figure out what is important.
Philosophizing aside, if you want to prioritize actions in OF, just drag them to the top of the list or use the "flag" feature to indicate the things you need to get done today. You really don't need a lot of tools for assigning priority if you're using the GTD framework.